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

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.

This implements the InternalPluginDriver interface in each driver, and
calls the cancellation fn for their respective eventers.

This fixes a per task goroutine leak during test suite execution.

libcontainer based drivers (e.g. exec, java) should default to running
processes as `nobody` [1]; but libcontainer treats empty user as `root`
in our case (either because of default or due to `root` being current
user).

[1] https://github.com/hashicorp/nomad/blob/94c28a4c6cc45f1b377d50a6bff2017b46c43d18/website/source/docs/job-specification/task.html.md#task-parameters

This is an unrecoverable error, so we should only do this check once,
rather than returning unhealthy constantly.

This PR plumbs the plugins done ctx through the base and driver plugin
clients (device already had it). Further, it adds generic handling of
gRPC stream errors.

Java driver supports non-linux targets, so LinuxResources fields isn't
guaranteed to be not-nil.

Task restoring works but dead tasks may be restarted

In tests:
* always cleanup (Kill) the harness
* don't call SetConfig more than once (a race)
* avoid cgroups to allow running without being root

from #4777 comments

Do not register mock_driver on release builds.

client: fingerprint driver plugins