minor comment changes, code rearrangement and field renaming

minor comment changes, code rearrangement and field renaming
to minimize diffs with future modifications.

The current implementation is problematic for the following reasons:
+ all insertions are O(N) because the event list does not have a tail
  pointer;
+ there is only a single lock protecting both session and users queues.
+ the implementation of the queue itself is not documented.
  I think i have figured it out, more or less, but am unclear on
  whether there is proper locking in place

The rewrite (which i have working locally) uses a tailq so insertions
are O(1), separate locks for the event and session queues, and has
a documented implementation so hopefully we can figure out if/where
bug exist.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@45753 65c4cc65-6c06-0410-ace0-fbb531ad65f3