There is not really a reason to stop tracking them, just because the
server says so. We might still want to show a users profile, etc.

This fixes spaces or stickers not showing up for old databases as well
as the wrong format of the state_keys db.

Also fixes some issues where nested transactions will poison the
verification cache.

fixes #636

Nested QEventLoops are scary. Ultimately we shouldn't use them, but I
have no better solution right now.

fixes #656

We share all keys with our devices and ones created by us to other
users.

As of 0db4d71e (Prevent edits of
unsent messages), messages that are edits of (or replies to) unsent
messages were not allowed. This change was made because otherwise
the edits were discarded due to use of txnid rather than mxid in the
"m.relates_to" object. Remove this restriction and fix the issue by
replacing txnid with mxid in all related events when the message is
sent (and we obtain mxid from the server).

Does not fix the read status yet, for that we need to compare read
receipts for all events after the last visible event.

Link opening only works on Linux for now.

See https://github.com/matrix-org/matrix-doc/pull/2312