add MessageQueue class

[bkueng]

When using Connection.add_message_callback from python, there's a potential for deadlocks:

• if the message receiving thread receives a message:
  • takes the _message_callback_mtx lock
  • calls back into python which takes the GIL lock
• if the main thread calls Connection.expect (or receive):
  • GIL lock is held
  • takes the _message_callback_mtx lock So both threads take the locks in opposite order, which can result in deadlocks.

To prevent this we could make sure libmav does not hold locks when calling back to python, unlock GIL when calling Connection.expect (and others), or ensure the receive thread does not call back into python. This patch implements the last solution by introducing an (async) message queue. It allows python code to receive and handle messages at well-defined points.

[ThomasDebrunner]

Thanks, I rebased to main and added feature to use python iterators and len functions.

queue = libmav.MessageQueue(client_conn)

# wait a bit and do other work

print(f"Have {len(queue)} messages in queue")  # will have some messages in queue when received

for message in queue:
    print(message.name)

print(f"Have {len(queue)} messages in queue") # will be empty after iterating through.