A Channel will now multiplex RPCs on a single managed connection. The
connection will attempt to reconnect on failure, and will close the
underlying transport connection if no RPCs have been made for a while.
Part of #5.
First stage of separating Connection from Channel. A Channel manages
multiple Connections, and chooses which Connection to send an RPC on.
In this change, the Channel still creates a Connection for each RPC.
Managing the Connection life-cycle comes in a later change.