Move initialization of the Port struct to the child process

Move initialization of the Port struct to the child process

In postmaster, use a more lightweight ClientSocket struct that
encapsulates just the socket itself and the remote endpoint's address
that you get from accept() call. ClientSocket is passed to the child
process, which initializes the bigger Port struct. This makes it more
clear what information postmaster initializes, and what is left to the
child process.

Rename the StreamServerPort and StreamConnection functions to make it
more clear what they do. Remove StreamClose, replacing it with plain
closesocket() calls.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi