Fixes https://github.com/matrix-org/synapse/issues/8238
Alongside the delta file, some changes were also necessary to the codebase to remove references to the now defunct `populate_stats_process_rooms_2` background job. Thankfully the latter doesn't seem to have made it into any documentation yet :)
* Fixup `ALTER TABLE` database queries
Make the new columns nullable, because doing otherwise can wedge a
server with a big database, as setting a default value rewrites the
table.
* Switch back to using the notifications count in the push badge
Clients are likely to be confused if we send a push but the badge count
is the unread messages one, and not the notifications one.
* Changelog
This is *not* ready for production yet. Caveats:
1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.
