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

mgw: Rename and move several get_conn funcs acting on endp object

These functions act on the endp object, hence prefix them with
mgcp_endp_* and move them to mgcp_endp.{c,h}.

Change-Id: Ifff01331db68998e9e23f99d8836d96022d550c2

mgw: mgcp_network.c: Simplify use of conn_rtp ptr

Change-Id: I5bab15fc793434173660769a8e60dae4ae4aa4c6

mgw: Clean up access to conn_rtp from conn

Add a new mgcp_conn_get_conn_rtp() and use it everywhere instead of
accessing deep structure fields.

Change-Id: Iee2c19598e9570ea3b1ceba3cdfd2a5f5be2c954

mgw: Avoid 2nd lookup of conn in endp during CRCX

There's no need to lookup (iterate over conns) for rtp_conn in endp, we
have it accessible in constant time.

While at it, simplify some accesses to the pointer after it.

Change-Id: I8316e76a1789d4a8eaa1eb89bdaf86e25dc0a1e8

cosmetic: mgw: iuup: Update comment

Change-Id: I6e1448bf7e3bb7454754147e2b2e01844c157500