Message lumps are saved to shared memory when t_relay() is called

Message lumps are saved to shared memory when t_relay() is called
the first time, instead of saving them by t_newtran(). The advantage is
that the SIP msg modifications that are made after t_newtran() are saved
as well, and they are propagated to failure route.
t_save_lumps() function is introduced, it can be used to force saving
the lumps before t_relay(), check the doc for details.
Fixes SER-303

The change can be reverted by uncommenting #define POSTPONE_MSG_CLONING
in sip_msg.h.