access channel locks through ast_channel_lock/unlock/trylock and not

access channel locks through ast_channel_lock/unlock/trylock and not
through ast_mutex primitives.

To detect all occurrences, I have renamed the lock field in struct ast_channel
so it is clear that it shouldn't be used directly.

There are some uses in res/res_features.c (see details of the diff)
that are error prone as they try and lock two channels without
caring about the order (or without explaining why it is safe).

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89293 65c4cc65-6c06-0410-ace0-fbb531ad65f3