One of the issues we noticed is that the partitioned topic
in push notification is heavy in traffic, as any user using a particular
mailserver will use that partitioned topic to register for PNs.
This commit moves from the partitioned topic to the personal topic of
the PN server, so it does not clash with other users that might happen
to have the same partitioned topic as the mailserver, resulting in long
sync times.
Another issue that will need to be addressed separately is that once you
send a message to a topic, because of the way how waku/whisper works,
you will have to register to that topic, meaning that you will receive
that data. Currently waku does not support unsubscribing from a topic
without logging in and out, so that needs also to be addressed.