Define and implement IncomingMailService

All the logic that currently lives inside mail.imap.fetch should be moved to a different service. To begin with, the module can be moved to mail.incoming.getmail or a module alike.

In here, we should also use a single, generic Parser for the incoming mail, and pass structured messages all along, instead of passing the raw strings.

(from redmine: created on 2014-11-12, closed on 2015-01-22, relates #6362 (closed), relates #6151 (closed))