Store thread ids for messages that we haven't seen yet

This allows us to thread messages even when we receive them out of
order, or never receive the root.

The thread ids for messages that aren't present but are referred to are
stored as metadata in the database and then retrieved if we ever get
that message.

When determining the thread id for a message we also check for this
metadata so that we can thread descendants of a message together before
we receive it.

Edited by Carl Worth <cworth@cworth.org>: Split this portion of the
commit from the earlier-applied portion adding test cases.