[MINOR] session: add the trk_conn_cnt ACL keyword to track connection counts

[MINOR] session: add the trk_conn_cnt ACL keyword to track connection counts

Most of the time we'll want to check the connection count of the
criterion we're currently tracking. So instead of duplicating the
src* tests, let's add trk_conn_cnt to report the total number of
connections from the stick table entry currently being tracked.

A nice part of the code was factored, and we should do the same
for the other criteria.