[FIX] OPW 577359: mail,fetchmail: more robust handling of incoming mails

[FIX] OPW 577359: mail,fetchmail: more robust handling of incoming mails

- Support inconsistent encoding specifications in
  mail bodies with crashing
- Mark messages as seen on IMAP servers only when they
  have actually been processed
- Properly commit after each incoming message, avoiding
  global rollback in case of later failure (during
  fetchmail cron task)
- Avoid crashing when importing mail without a proper
  From header

Note: this fix depends on a corresponding server fix
(revno: 4248
rev-id: odo@openerp.com-20120727134402-vafp3wqlwnjsxntk)

bzr revid: odo@openerp.com-20120727135007-awudey1lkqtzoaot