ejabberd receivers were meant to serve connections from frontends
to backends. However, this approach was not popular and frontend
related code was removed in previous releases. Now, ejabberd receiver's
code was also removed, making the code shorter and cleaner. Also, in
stress tests ejabberd now handles load more robustly, without
c2s processes overload (even with disabled shapers).
ejabberd_socket.erl is renamed to xmpp_socket.erl: it's supposed to
be finally moved into stand-alone xmpp library.
