Allow mypy client-server communication via TCP sockets

Feature

It would be useful to allow the user to define a custom TPC host:port pair and have the client communicate with a remote mypy server listening to that address.

mypy already uses a client-server architecture with the server being a daemon running in the same process space and communicating with the client via UNIX sockets (named pipes in win32) so this would not require large changes AFAIK.

Pitch

This feature would enable developers using Docker to run a mypy server inside the container, but be able to call the mypy command from the host machine just as easily as when everything runs in the same host.

This would make mypy functional to several existing editor and IDE integrations, as the call is transparently routed to the server inside the container.

Start by tracing mypy's existing client-server communication over UNIX sockets or Windows named pipes. Determine where a custom TCP host:port would be configured and how the remote server connection should work, including the Docker use case. Done means the client can communicate with a remote mypy server while existing local communication remains functional.