This sprinkles a few mutexes around the plugin interface so that the
olLaunchKernel CTS test now passes when ran on multiple threads.

Part of this also involved changing the interface for device synchronise
so that it can optionally not free the underlying queue (which
introduced a race condition in liboffload).

Add a queue query that (if possible) reports whether the queue is empty

Add a device function to check if a device queue is empty. If liboffload
tries to create an event for an empty queue, we create an "empty" event
that is already complete.

This allows `olCreateEvent`, `olSyncEvent` and `olWaitEvent` to run
quickly for empty queues.

`MAX_WORK_GROUP_SIZE` now represents the maximum total number of work
groups the device can allocate, rather than the maximum per dimension.
`MAX_WORK_GROUP_SIZE_PER_DIMENSION` has been added, which has the old
behaviour.

Instead using strings to look up device information (which is brittle
and slow), use the new tags that the plugins specify when building the
nodes.

Rather than having every "enqueue"-type function have an output pointer
specifically for an output event, just provide an `olCreateEvent`
entrypoint which pushes an event to the queue.

For example, replace:
```cpp
olMemcpy(Queue, ..., EventOut);
```
with
```cpp
olMemcpy(Queue, ...);
olCreateEvent(Queue, EventOut);
```

This function causes a queue to wait until all the provided events have
completed before running any future scheduled work.

This more closely matches the nomenclature used by CUDA, AMDGPU and
the plugin interface.

When creating a new symbol, check that it already exists. If it does,
return that pointer rather than building a new symbol structure.