Support Unix Domain Sockets

[caleblloyd]

"In Unix environment with Mono, this can be a fully qualified path to MySQL socket filename. With this configuration, the Unix socket will be used instead of TCP/IP socket. Currently only a single socket name can be given so accessing MySQL in a replicated environment using Unix sockets is not currently supported."
#117 proposes a theory that TCP buffering may be causing slowdowns. Implementing Unix Domain Sockets will allow for benchmarking the library while circumventing the TCP layer, giving a better idea of the performance potential of the library without TCP.

[caleblloyd]

I have a WIP of this here: caleblloyd@315e57b

I am stuck at trying to get Socket.Connect to connect to a Unix Domain Socket. See my question on Stack Overflow for more information.

[caleblloyd]

I solved connecting to a Unix Domain Socket by copying UnixEndPoint.cs from the Mono.Posix package.

New working WIP is at caleblloyd@1ac303e. Initial performance tests do not result in a much higher throughput than TCP, so retransmissions may not be what is holding performance down as originally thought in #117. Retransmissions may just be a side effect of pauses caused by Garbage Collection or context switching.

I'm going to wait for #115 to land before opening a PR for Unix Domain Sockets to avoid introducing merge conflicts.

[bgrainger]

Merged #115 (to unblock you); will fix minor code issues with it later.