[FIX] mass_mailing: fixes

- fixed keeping the original message for routing, only when choosing to reply in the
original thread (notification=True)
- auto delete sent emails explicitely
- mail_thread: routing: fixed replies always choosen even when replying to emails
with a specified reply_to (using ref_match in the algorithm)
- mail_thread: routing: instead of exclusive routing heuristics, use each case
as a fallback of the previous.