delete keys when deleting backup versions

Remove repeated calls to config.stats_enabled.

Fix /keys/query API on workers.

The necessary getters were added only to the master store and not the
worker stores.

Turns out that fetching variables from the config object is expensive,
so doing it once at startup avoids unnecessary work.

Reduce debug logging overhead

Make user stats less verbose

Using % will cause the string to be generated even if debugging
is off.

This avoids calculating __repr__ unless we are going to log.

I believe that string formatting ~10-15 sized events will
take a proportion of CPU time.

Move schema delta files to the correct data store.

Add new boolean column to synapse_port_db

They were put in the global schema delta directory due to a bad merge.

The expected use case is to suppress MAU limiting on small instances

Fix import paths in synapse_port_db

Make `synapse_port_db` correctly create indexes in the PostgreSQL database, by having it run the background updates on the database before migrating the data.

To ensure we're migrating the right data, also block the port if the SQLite3 database still has pending or ongoing background updates.

Fixes #4877 

Add a linting script that enforces all boolean values in the default config be lowercase.

This has annoyed me for a while so I decided to fix it.