Better interface for Rpc.Connection.{client,with_client}

It should accept an `'addr where_to_connect` instead of forcing the use of a TCP connection. Locally, an unix connection instead of a TCP one would be more efficient.

Start by locating the Rpc.Connection.client and Rpc.Connection.with_client entry points and inspecting their current TCP-only interfaces. Trace how the connection type is used locally, then define the scope of accepting an 'addr where_to_connect value; done means both interfaces support non-TCP connections such as Unix connections without breaking existing callers.

Written by the indexing model from the issue text.