res_mwi_external: Clear the stasis cache entry when the external MWI is deleted.

One of the things missing when external MWI support was added was the
ability to clear the stasis cache entry of deleted external MWI mailboxes.

Review: https://reviewboard.asterisk.org/r/3325/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410555 f38db490-d61c-443f-a65b-d21fe96a405b
ARI/bridges: Forward Playback/Recording Started/Finished to bridge topic

(closes issue ASTERISK-23444)
Reported by: Ben Merrills
Review: https://reviewboard.asterisk.org/r/3340/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410558 f38db490-d61c-443f-a65b-d21fe96a405b