Define a shim protocol

[sameo]

Define a simple, TCP based protocol for communicating with container shims

The protocol would allow shims to send over a single TCP socket the following commands:

• Connection/Disconnection: For a shim to explicitly connect and pass an authentication token to the proxy.
• I/O: Send stdin streams to a given container.
• Signals: All signals, including SIGWINCH.
• Logs: Shim logs could be aggregated by the proxy for easier debugging.

And to receive the following commands:

• I/O: Receive stdout/stderr streams from a given container