stasis: Add methods to allow for synchronous publishing to subscriber

This patch adds an API call to Stasis that allows a publisher to publish a
stasis message that will not return until a specific subscriber handles the
message. Since a subscriber can have their own forwarding topic which orders
messages from many topics, this allows a publisher who knows of that subscriber
to synchronize to that subscriber regardless of the forwarding relationships
between topics.

This is of particular use for dialplan applications that need to synchronize
on a particular subscriber's handling of a message.

(issue ASTERISK-22884)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3099/
........

Merged revisions 405311 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405313 65c4cc65-6c06-0410-ace0-fbb531ad65f3