Incoming messages go to INBOX or UNVERIFIED, depending on whether we
have the relevant keys in cache.

All messages have a local ID formed of eight hex digits (such as "1234ABCD").
Messages generated locally have a message ID based on the local ID.
The only messages which might not have a message ID are the ones in ERRORS
(because they might be malformed). Messages can be looked up by their
local ID or their message ID.

States of a message:
    - UNVERIFIED
        The message has arrived, but we haven't managed to verify it yet.
        In order to verify it, we need to have copies of some public keys,
        which we don't currently have cached.

        When we receive a key, we check to see which messages are waiting
        on it. If any of them are waiting on no other keys, then we
        run verification.

    - INBOX
        The incoming message is verified, and is currently running
        through hooks.

    - RECEIVED
        The incoming message has been delivered, but we're keeping a copy in
        the vault.

    - ERRORS
        Something went wrong with the message. The message might be
        an incoming message that failed verification, an outgoing
        message to a user that doesn't exist, a malformed message, and so on.
        The wrapper dict will have an extra key "error" containing
        a machine-readable explanation of why the message is in ERRORS.

    - OUTBOX
        The message is going to be sent, or at least we intend so,
        but we need some remote items which we don't currently have cached.

        When we receive an item, we check to see which messages are
        waiting on it. If any of them are waiting on no other items,
        we sign the message and send it.

    - SENT
        The message has been sent successfully.

    - GONE
        Anything in here represents a message that did exist, and has
        been deleted, but we didn't want to forget about it completely.
        They may have a "tombstone" key for the web server's benefit.