Make `CodecContext.decode` non-blocking?

[oakaigh]

Currently, CodecContext.decode blocks until the packet is fully decoded; For containers with multiple streams, this isn't ideal because streams can be decoded in parallel.

It would be useful if CodecContext.send_packet and CodecContext.recv_frame are available - or better, CodecContext.decode_async, which returns a concurrent.futures.Future. The underlying implementation is already async so adding support is trivial. The current (and obvious) workaround is to create the CodecContext in another thread but that's just unnecessary.