Schema change upgrade instructions, schema 21

This upgrade shouldn’t be substantially different than past upgrades, now that we’ve fixed a few bugs with the process. To upgrade:

1. Make sure your REPLICATION_TYPE setting is RT_SLAVE and your DB_SCHEMA_SEQUENCE is set to 20 in lib/DBDefs.pm.
2. Ensure you’ve replicated up to the most recent replication packet available with the old schema. (if you’re not sure, run ./admin/replication/LoadReplicationChanges and see what it tells you).
3. Take down the web server running MusicBrainz, if you’re running a web server.
4. Turn off cron jobs if you are automatically updating the database via cron jobs.
5. Switch to the new code with git fetch origin followed by git checkout schema-change-20-to-21
6. Run ./upgrade.sh (or carton exec -Ilib -- ./upgrade.sh if you’re using carton, with very old setups).
7. Set DB_SCHEMA_SEQUENCE to 21 in lib/DBDefs.pm
8. Turn cron jobs back on, if needed.
9. Restart the MusicBrainz web server, if applicable. It’s also recommended you restart memcached.

That’s it! The only real difference from the past is the specific tag to be used: schema-change-20-to-21, which is a couple of fix-up commits past the regular release tag.