Add a new factory method instead of direct instatiation of the threaded background worker.
This allows for easy extension to other types of workers, such as the upcoming task-based async worker.

GH-4578

Add a new flush_async method to worker ABC. This is necessary because the async transport cannot use a
synchronous blocking flush.

GH-4578

Move the flush_async down to the concrete subclass to not break existing testing. This makes sense,
as this will only really be needed by the async worker anyway and therefore is not shared logic.

GH-4578

Coroutines have a return value, however the current function signature for the worker methods does not
accomodate for this. Therefore, this signature was changed.

GH-4578

GH-4578

Refactor the flush method in the async worker to use the async_flush coroutine.

GH-4581

…unctions

Add a check to see wheter callbacks are awaitable coroutines or functions, as coroutines need to be awaited.

GH-4581

…coroutines

Coroutines do not return None, therefore it is necessary to consider this in the callback parameter of the worker. Previously,
only callbacks with return Type None were accepted.

GH-4581

GH-4581

GH-4581

Enable concurrent callbacks on async task worker by firing them as a task rather than awaiting them. A done callback handles the necessary queue and exception logic.

GH-4581

Changed kill to also use the _TERMINATOR sentinel, so the queue is still drained to this point on kill instead of cancelled immediately. This should also fix potential race conditions with flush_async.

GH-4581

Add proper type annotation to worker task list to fix linting problems

GH-4581

Refactor worker implementation to simplify callback processing, fix pending calculation and improve queue initialisation.

GH-4581

The flush method in the transport enqueues a sync callback for the worker, therefore the check needs to be here after all.

GH-4581