Add channel lock protection around translation path setup.

Most callers of ast_channel_make_compatible() happen before the channels
enter a two party bridge.  With the new bridging framework, two party
bridging technologies may also call ast_channel_make_compatible() when
there is more than one thread involved with the two channels.

* Added channel lock protection in set_format() and
ast_channel_make_compatible_helper() when dealing with the channel's
native formats while setting up a translation path.

* Fixed best_src_fmt and best_dst_fmt usage consistency in
ast_channel_make_compatible_helper().  The call to
ast_translator_best_choice() got them backwards.

* Updated some callers of ast_channel_make_compatible() and the function
documentation.  There is actually a difference between the two channels
passed in.

* Fixed the deadlock potential in res_fax.c dealing with
ast_channel_make_compatible().  The deadlock potential was already there
anyway because res_fax called ast_channel_make_compatible() with chan
locked.

(closes issue ASTERISK-22542)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/2915/
........

Merged revisions 401239 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@401240 65c4cc65-6c06-0410-ace0-fbb531ad65f3