add cumulative count of sessions from switch startup.  Resolves http://jira.freeswitch.org/browse/FSCORE-18.  Updated patch to change data type of session_id to avoid rollover (wouldn't that be nice) and added - 1 to accurately reflect count.  Thanks bkw for the patch.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4999 d0543943-73ff-0310-b7d9-9358b9ac24b2