autocreate: Redesigned autocreate plugin to create/subscribe mailboxes lazily. Mailboxes are listed as if they existed, but they're not created until the mailbox is opened for the first time. Mailboxes are autosubscribed only during mailbox creation, so subscription file isn't unnecessarily being read all the time either. This lazy creation also means that if autocreate mailbox is removed from configuration and it was never opened by the client, it was also never physically created.