Improve the handling of notifications

The aim is to put some additional filtering in place to better stop floods of old notifications. This is achieved with a couple of new filters and better tracking of old notifications.

- Make sure to paginate through all notification on initialization to ensure they are all added to old notifications. While we were not previously putting a limit on the number of returned notifications the server can and will do this when there are a very large amount.
- Find the newest timestamp for each connection on initialization and don't post any notifications with an earlier timestamp.
- Track old notifications on a per-connection basis.

Closes network/neochat#358 and network/neochat#423