Fix a crash when logging out from the AMI and avoid astobj2 warning messages.

When the user logout the session was being destroyed twice and the file
descriptor was being closed twice. The sessions reference counter wasn't
used in a proper way.
The 'mansession' structure was being treated as an astobj2 and we were
calling ao2_lock/ao2_unlock causing astobj2 report a warning message and
not locking the structure.
Also we were using an ugly naming convention 'destroy_session',
'session_destroy', 'free_session', ... all this "duplicated" code was merged.

(closes issue #14974)
Reported by: pj
Patches:
      manager.diff2 uploaded by eliel (license 64)
      Tested by: dhubbard, eliel, mnicholson

(closes issue #15088)
Reported by: eliel

Review: http://reviewboard.asterisk.org/r/248/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@194060 65c4cc65-6c06-0410-ace0-fbb531ad65f3