Thread lookup

Change how we resolve thread from a message. Logic for thread create or update is now in UserMessageDelivery.process method

Now a parsed message must have a lookup_sequence method that give order for thread lookup, UserMessageDelivery will handle lookup to find an existing thread or create it.

We can lookup by : 
- an external thread id (not used in normal mail)
- an external parent message id (Reply-To for mail)
- a recipient name (a list or from recipient)