Log lines which include an error should use the full term "error"
as the context key. This provides consistency across the codebase
and avoids a Go style which operators might not be aware of.

The QEMU driver can take an optional `graceful_shutdown` configuration
which will create a Unix socket to send ACPI shutdown signal to the VM.

Unix sockets have a hard length limit and the driver implementation
assumed that QEMU versions 2.10.1 were able to handle longer paths. This
is not correct, the linked QEMU fix only changed the behaviour from
silently truncating longer socket paths to throwing an error.

By validating the socket path before starting the QEMU machine we can
provide users a more actionable and meaningful error message, and by
using a shorter socket file name we leave a bit more room for
user-defined values in the path, such as the task name.

The maximum length allowed is also platform-dependant, so validation
needs to be different for each OS.

When a QEMU task is recovered the monitor socket path was not being
restored into the task handler, so the `graceful_shutdown` configuration
was effectively ignored if the client restarted.

Add boolean 'guest_agent' config option for QEMU driver, which will
create the socket file for the QEMU Guest Agent in the task dir when
enabled.

This test exercises upgrades between 0.8 and Nomad versions greater
than 0.9. We have not supported 0.8.x in a very long time and in any
case the test has been marked to skip because the downloader doesn't
work.

The QEMU driver allows arbitrary command line options, but many of
these options give access to host resources that operators may not
want to expose such as devices. Add an optional allowlist to the
plugin configuration so that operators can limit the resources for
QEMU.

* Enable network namespaces for QEMU driver
* Add CHANGELOG entry

Co-Authored-By: Tim Gross <tgross@hashicorp.com>
Co-Authored-By: Seth Hoenig <shoenig@hashicorp.com>

This fixes few cases where driver eventor goroutines are leaked during
normal operations, but especially so in tests.

This change makes few modifications:

First, it switches drivers to use `Context`s to manage shutdown events.
Previously, it relied on callers invoking `.Shutdown()` function that is
specific to internal drivers only and require casting.  Using `Contexts`
provide a consistent idiomatic way to manage lifecycle for both internal
and external drivers.

Also, I discovered few places where we don't clean up a temporary driver
instance in the plugin catalog code, where we dispense a driver to
inspect and validate the schema config without properly cleaning it up.

When an allocation runs for a task driver that can't support volume mounts,
the mounting will fail in a way that can be hard to understand. With host
volumes this usually means failing silently, whereas with CSI the operator
gets inscrutable internals exposed in the `nomad alloc status`.

This changeset adds a MountConfig field to the task driver Capabilities
response. We validate this when the `csi_hook` or `volume_hook` fires and
return a user-friendly error.

Note that we don't currently have a way to get driver capabilities up to the
server, except through attributes. Validating this when the user initially
submits the jobspec would be even better than what we're doing here (and could
be useful for all our other capabilities), but that's out of scope for this
changeset.

Also note that the MountConfig enum starts with "supports all" in order to
support community plugins in a backwards compatible way, rather than...

drivers: always initialize taskHandle.logger

Looks like the RecoverTask doesn't set taskHandle.logger field causing
a panic when the handle attempts to log (e.g. when Shutdown or Signaling
fails).

This ensures that `port_map` along with other block like attribute
declarations (e.g. ulimit, labels, etc) can handle various hcl and json
syntax that was supported in 0.8.

In 0.8.7, the following declarations are effectively equivalent:

```
// hcl block
port_map {
  http = 80
  https = 443
}

// hcl assignment
port_map = {
  http  = 80
  https = 443
}

// json single element array of map (default in API response)
{"port_map": [{"http": 80, "https": 443}]}

// json array of individual maps (supported accidentally iiuc)
{"port_map: [{"http": 80}, {"https": 443}]}
```

We achieve compatbility by using `NewAttr("...", "list(map(string))",
false)` to be serialized to a `map[string]string` wrapper, instead of using
`BlockAttrs` declaration.  The wrapper merges the list of maps
automatically, to ease driver development.

This approach is closer to how v0.8.7 implemented the fields [1][2], and
despite its verbosity, seems to perserve 0.8.7 behavior in hcl2.

This is only required for built-in types that have backward
compatibility constraints.  External drivers should use `BlockAttrs`
instead, as they see fit.

[1] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/docker.go#L216
[2] https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/docker.go#L698-L700

I wanted a different color bikeshed, so I get to paint it

plugins/driver: update driver interface to support streaming stats

client/tr: use streaming stats api

TODO:
 * how to handle errors and closed channel during stats streaming
 * prevent tight loop if Stats(ctx) returns an error

drivers: update drivers TaskStats RPC to handle streaming results

executor: better error handling in stats rpc

docker: better control and error handling of stats rpc

driver: allow stats to return a recoverable error

This PR fixes various instances of plugins being launched without using
the parent loggers. This meant that logs would not all go to the same
output, break formatting etc.

Re-export the ResourceUsage structs in drivers package to avoid drivers
directly depending on the internal client/structs package directly.

I attempted moving the structs to drivers, but that caused some import
cycles that was a bit hard to disentagle.  Alternatively, I added an
alias here that's sufficient for our purposes of avoiding external
drivers depend on internal packages, while allowing us to restructure
packages in future without breaking source compatibility.