tmx: t_precheck_tran() - light filtering of retransmissions

- keep track of processed messages per process to catch retransmissions
  in early phase, to avoid simultaneous processing until transaction is
  created
kamailio.cfg: use t_precheck_trans() to catch retransmissions faster
tmx: documentation for t_precheck_trans()
tmx: precheck_trans - new parameter to enable/disable pre-transaction check

- enable/disable usage of t_precheck_trans() - don't init internal
  structures
tmx: documented precheck_trans parameter