bridge_builtin_features: Add missing channel locks around ast_get_chan_features_gener...

bridge_builtin_features: Add missing channel locks around ast_get_chan_features_general_config().

The feature_automonitor() and feature_automixmonitor() functions were not
locking the channel around ast_get_chan_features_general_config().
Accessing the channel datastore list without the channel locked is a good
way to corrupt the list or follow the pointer chain into oblivion.
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Merged revisions 426531 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 426552 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@426553 65c4cc65-6c06-0410-ace0-fbb531ad65f3