Move the code to purge stale sessions to a function,

to simplify the body of the main loop of the accepting thread.
Rename purge_unused() to purge_events() so one knows what the
function does.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48081 65c4cc65-6c06-0410-ace0-fbb531ad65f3