res_parking: Minor tweaks.

* Use ast_bridge_channel_lock()/ast_bridge_channel_unlock() instead of
ao2_lock()/ao2_unlock() for struct ast_bridge_channel variables.

* Use ast_copy_string() instead of inlining it.

* Remove an already done TODO comment.

* Some whitespace tweaks.
........

Merged revisions 411638 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411639 65c4cc65-6c06-0410-ace0-fbb531ad65f3