[tunnelbroker] Implement lazy AMQP channel wrapper

Summary:
This cancels minor quality regression introduced earlier in D13604.

• For websocket, we store the channel in the WebsocketSession struct and reuse it unless it's broken
• However for gRPC, after opted out of channel pool, a new short-lived channel is created for every single call, which is against what RabbitMQ recommends.

Created a simple wrapper that lazily-initializes the channel upon first use, and creates a new one only if the previous was closed.

Depends on D13611

Test Plan: Added some additional logging and ran Commtest

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13621