Prepare executor to handle streaming exec API calls that reuse drivers protobuf
structs.

Reverts hashicorp/nomad#5433

Apparently, channel communications can constitute Happens-Before even for proximate variables, so this syncing isn't necessary.

> _The closing of a channel happens before a receive that returns a zero value because the channel is closed._
https://golang.org/ref/mem#tmp_7

exitState is set in `wait()` goroutine but accessed in a different
`Wait()` goroutine, so accesses must be synchronized by a lock.

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.

IOPS have been modelled as a resource since Nomad 0.1 but has never
actually been detected and there is no plan in the short term to add
detection. This is because IOPS is a bit simplistic of a unit to define
the performance requirements from the underlying storage system. In its
current state it adds unnecessary confusion and can be removed without
impacting any users. This PR leaves IOPS defined at the jobspec parsing
level and in the api/ resources since these are the two public uses of
the field. These should be considered deprecated and only exist to allow
users to stop using them during the Nomad 0.9.x release. In the future,
there should be no expectation that the field will exist.

 * Added protobuf spec for executor
 * Seperated executor structs into their own package

This creates a drivers/shared/structs package and moves the buffer size
checks into it.

added wrapper struct for plugin.ReattachConfig to better handle serialization

Driver plugin framework to facilitate development of driver plugins.

Implementing plugins only need to implement the DriverPlugin interface.
The framework proxies this interface to the go-plugin GRPC interface generated
from the driver.proto spec.

A testing harness is provided to allow implementing drivers to test the full
lifecycle of the driver plugin. An example use:

func TestMyDriver(t *testing.T) {
    harness := NewDriverHarness(t, &MyDiverPlugin{})
    // The harness implements the DriverPlugin interface and can be used as such
    taskHandle, err := harness.StartTask(...)
}

* client/executor: refactor client to remove interpolation

* executor: POC libcontainer based executor

* vendor: use hashicorp libcontainer fork

* vendor: add libcontainer/nsenter dep

* executor: updated executor interface to simplify operations

* executor: implement logging pipe

* logmon: new logmon plugin to manage task logs

* driver/executor: use logmon for log management

* executor: fix tests and windows build

* executor: fix logging key names

* executor: fix test failures

* executor: add config field to toggle between using libcontainer and standard executors

* logmon: use discover utility to discover nomad executable

* executor: only call libcontainer-shim on main in linux

* logmon: use seperate path configs for stdout/stderr fifos

* executor: windows fixes

* executor: created reusable pid stats collection utility that can be used in an executor

* executor: update fifo.Open calls

* executor: fix build

* remove executor from...

Sending the Ctrl-Break signal to PowerShell <6 causes it to drop into
debug mode. Closing its output pipe at that point will block
indefinitely and prevent the process from being killed by Nomad.

See the upstream powershell issue for details:
https://github.com/PowerShell/PowerShell/issues/4254

This commit changes the force closing of the stdout/stderr file
descriptor from closing immediately to being closed after a grace
period. This allows the created process to close its own file and allows
copying of the data.

- Set CREATE_NEW_PROCESS_GROUP for Windows subprocess.
- Ensure we only kill actual process that need to.

Should help with debugging issues like #3971