Refactor sync API demultiplexing logic

Demultiplexing in the sync API had become quite tangled,
with the foreground thread responsible for allocating
multiplexing IDs and receive queues, while the background
thread handling the actual demultiplexing process.

Now the sync API is using async queues for its demultiplexing,
move multiplexing ID and queue management entirely to the
background thread.

Author: ncoghlan