Split the large client/server.dart files into smaller pieces. This is in
preparation for splitting the HTTP/2 dependencies into a separate file
and make it easier to implement other transports.
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.
Added an example showing how to send/receive custom metadata, and handle
call cancellation.
Implemented the remaining parts of metadata and cancel handling.
Addresses part of #8 and #12.
Michael Thomsen
Jakob Andersen