Prevent access to mailboxes while they are being moved

Since our automation can no longer merge mailboxes reliably now that they are encrypted (as a result of a limitation with Dovecot), it needs instead to move mailboxes as a whole. To support this we need to prevent email delivery and access while the mailbox is being moved.

Still missing the Postfix side: note that we can't make a generic LDAP query that says "host != originalHost", so we'll have to implement this on the postfix-delivery instance (which has a well-defined shard_id) so we can say "host=shard_id AND originalHost=shard_id". This way any incoming emails should end up queued inside our systems, which is fine.