Requested on gitter.

The unix socket can be a reasonably way to communicate with a CockroachDB node when running on the same machine. We added it a while back for easier testing (easier than dynamic port allocation) but have never advertised or documented it. The flag (--socket) is currently hidden.

We should consider surfacing the flag in the CLI and documenting it. We first need to add some additional testing for it, including exercising in test clusters.

CC: @bdarnell for thoughts.