Changes

mgw: Rename and move code freeing endp connection

All the code to free conns in an endpoint is really entanged, which also
makes it inefficient in several places because the list of conns is
iterated twice, or even ~N*2.

* Move code freeing the conn object to its own mgcp_conn_free()
  function.
* Rename functions acting on endp to be mgcp_endp_* prefix, and move
  them to mgcp_endp.{c,h}.
* Remove old mgcp_conn_free(endp, id) since it's not really needed
  anymore.

Change-Id: I9a87a07699dd8aa7856d5036e9b95a4ddf699a15