If we update the address after determining the sender address is
different, we also need to refresh the identity state in order to use
the right encryption keys.
* Only call `RemoveOnFail` on error. It was also called on success.
* Add some logging to track send times.
* Fix non-deterministic hash from `rfc8222.GetMessageHash`.
https://github.com/ProtonMail/gluon/pull/395
Refactor code to isolate the SMTP functionality in a dedicated SMTP
service for each user as discussed in the Bridge Service Architecture
RFC.
Some shared types have been moved from `user` to `usertypes` so that
they can be shared with Service and User Code.
Finally due to lack of recursive imports, the user data SMTP needs
access to is hidden behind an interface until the User Identity service
is implemented.
