[CLEAN] mail: clean mail gateway code

The purpose of this commit is to clean a bit the code related to email
routing in the mailgateway: message_route and message_route_verify.

message_route is now clearly separeted into two categories. First one
handles emails considered as answers to existing threads. Second one
handles emails that have to create new threads, either through alias or
default behavior of the mailgateway.

message_route_verify has been a bit facotrized to simplify the various
overrides that appeared lately in the codebase.

This commit only contains code cleaning. No functional change should occur
due to this commit.