Class Delete.

Deletes an existing mailbox (RFC 3501 section 6.3.4)

Mailboxes cannot be deleted until they're empty of messages and

(Really deletes? What happens to the mail there?)

RFC 2180section 3 is tricky. For the moment we disallow DELETE of an active mailbox. That's not practical to do on a cluster, so we'll need to think of a better policy.