wip: Add Unix socket support

[PaulusParssinen]

This PR adds initial unix socket support to Garnet. The unix domain socket path can be specified with --unixsocket switch. Because string address, int port pair was used everywhere, this change required a lot of tiny changes in testing and networking logic. There is more clean-up work left in some parts of the code, e.g. using IPEndPoint to represent cluster nodes instead of the (string address, int port) tuple.

I'm leaving implementation of --unixsocketperm switch to be done as a follow-up. I'm also for now making the server initialization throw NotImplementedException if user attempts to specify unix socket listener in cluster mode. Supporting cluster mode would need bit more work to have two different types of listener endpoints active.

TODO:

• Add some tests before marking PR ready
• more testing

Contributes to #682