Refactor ast_bridge_features struct.

* Reduced the number of hook containers to just dtmf_hooks,
interval_hooks, and other_hooks.  As a result, several functions dealing
with the different hook containers could be combined.

* Extended the generic hook struct for DTMF and interval hooks instead of
using a variant record.

* Merged the special talk detector hook into the other_hooks container.

* Replaced ast_bridge_features_set_talk_detector() with
ast_bridge_talk_detector_hook().

(issue ASTERISK-22107)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@395322 65c4cc65-6c06-0410-ace0-fbb531ad65f3