Refactor: +SocketMixin

[digitalextremist]

Right now, socket optimizations happen per-connection vs. per-server, and SSLServer is not covered... which this refactor resolves by using SocketMixin across both servers, and going full per-server vs. per-connection.

[digitalextremist]

This might conflict some with #113. Not sure what order wouldn't produce a conflict.

[stouset]

These magic constants could use a comment.

[digitalextremist]

RUBY_PLATFORM only, or all the Socket:: constants and the symbols also?

The Socket constants are notoriously hard to find documentation on, but RUBY_PLATFORM will be set to whatever the OS is, and only Linux supports the behavior expected of #optimize_socket

[digitalextremist]

This seems like a good time to also add a Keep-Alive timeout option, rather than having a hard-coded default, which also I cannot seem to find. Seems like there might not even be a default, and I see threads staying open purely for Keep-Alive now that pipelining is supported. Do you know where I can add this @tarcieri or @halorgium?

[tarcieri]

@digitalextremist the timeout situation is a bit dire. I'm in the process of refactoring all the requests and response state machines into Celluloid::FSMs