start introducing hooks for reference counts on dialog descriptors.

start introducing hooks for reference counts on dialog descriptors.
This commit is, for all practical purposes, a no-op, as it only
introduces the dialog_ref() and dialog_unref() methods, and uses them
in a few places (not all the places where they would be needed).

The goal is to start annotating the code with these calls, so the transition
to a proper container will be easier.

Nothing to backport.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@77647 65c4cc65-6c06-0410-ace0-fbb531ad65f3