Log Message:
Update to Dovecot 2.0.15.

+ doveadm altmove: Added -r parameter to move mails back to primary storage.
- v2.0.14: Index reading could have eaten a lot of memory in some situations
- doveadm index no longer affects future caching decisions
- mbox: Fixed crash during mail delivery when mailbox didn't yet have GUID
  assigned to it.
- zlib+mbox: Fetching last message from compressed mailboxes crashed.
- lib-sql: Fixed load balancing and error handling when multiple hosts are
  used.