stasis_channels.c: Misc cleanup.

* Use current OBJ_SEARCH_xxx defines instead of the deprecated versions.

* Fix hash_cb and cmp_cb container functions to correctly use the
OBJ_SEARCH_xxx values.

* Remove incorrect usage of CMP_STOP.  Most uses in the system have no
effect.  This allows the collapse of channel_role_single_cmp_cb() and
channel_role_multi_cmp_cb() into channel_role_cmp_cb().

* Remove unnecessary usage of RAII_VAR().

Change-Id: I02c405518cab22aa2a082b61e2353bf7cd629a70