Add replacement connection handler.

This handler and the added fields are not yet in use but will
be called instead of the current loop in main in order to handle
connections with clients and ships more cleanly.
Utilize new fields in BANANA in packet handlers.

Technically send_size will always be 0 by the time this function
is called for a particular client, but this is done here for
consistency with what all of the other packet handlers will do.
Adjust receiving expectations from 4 bytes to 8.

This will need to be adjusted later (and really a constant used
instead).
Move unimplemented connection handling into function and out of main.

This centralizes everything and now it's clearer what functionality
still needs to be implemented and how it fits in along with the other
code. Also reduces fragmentation and having to jump back and forth between
the new function and the old main loop.