Support socket control messages for rpc calls

[cpuguy83]

I'd like to be able to have rpc's that can pass file descriptors over transports which support it.

Today there is some support for an interceptor with a custom handshaker to pass along unix rights messages on connection initiation.
The method of passing file descriptors is similar (same mechanism, different message).

The difference here is it would need to be supported per request instead of at connection time (I think), so I'm not sure we'd have access to a handshaker.

[cpuguy83]

Draft implementation: #75