Fixes deadlocks occuring in chan_agent due to r335976

Fixes deadlocks occuring in chan_agent due to r335976

Bad locking order was added to chan_agent to prevent segfaults from having no locking
in a patch by irroot. This patch addresses the bad locking order by releasing locks before
getting the right locking order to stop deadlocks from occuring when doing multiple
interactions with agents.

(closes issue ASTERISK-19285)
Reported by: Alex Villacis Lasso
Review: https://reviewboard.asterisk.org/r/1708/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@353999 65c4cc65-6c06-0410-ace0-fbb531ad65f3