A more complex algorithm was used to achieve consensus between server
and client lists of notifications. This commit uses a different and
more simple approach that ignores order, but updates read status of
existing notifications and removes stale notifications.
When the client received a new notification, it prioritized only PM
notifications instead of maintaining the priority order. Later, the
check for missing notification deleted all notifications that were
in the wrong order because it could not match the IDs.
The correct order puts high_priority AND unread notifications first.
Low priority or read notifications (including high priority, but read
notifications) come after.
