We don't get notified for every message. Sometimes we only get a read
receipt for the newest message, which means old read receipts accumulate
in the database. This least to some considerable CPU overhead, when
checking if the timeline should be notified for new read receipts.
Instead just always notify, since that has far less overhead in the
worst case and doesn't need complicated cache cleanup.

The old pending_receipts db is not removed for now. It should still have
minimal storage overhead and we don't have a good mechanism for cache
format upgrades atm.

i was confused when a room with me and two friends showed as "user1" and
3 others.

This may increase the db size by a factor of 1000 in the worst case and
it will need some fixes, when we decide to not show some events in the
timeline, but it should work for now.

Only consider the room as unread, if there is a message in the room and
only consider message and sticker events as messages.

fixes #65