Add pluggable transport layer with RabbitMQ backend

[mikelangmayr]

Issue 17

• extract shared session logic (RequestSession, RequestServer, PublishSession, SubscribeSession) into transport/session.py
• simplify ZMQ transport to use the shared session layer and protocol.wire codec
• add RabbitMQ transport (transport/rabbitmq/) implementing the same interfaces via pika
• backend selection via MKTL_TRANSPORT env var (default: zmq)
• pika is an optional dependency: pip install mKTL[rabbitmq]

[klanclos]

I've only done a quick read, but it looks like there are some 'import zmq' references being removed by this pull request-- those should already be eliminated by #24. Is this out of sync with the main branch?

[prkrtg]

We're off by a few commits, but a rebase will address that. The focus of this PR is the transport change.