Start background tasks after we fork the process (daemonize) (#18886)

Spawning from https://github.com/element-hq/synapse/pull/18871

[This change](https://github.com/element-hq/synapse/pull/18871/commits/6ce2f3e59d6dcd8a798552a087414cf137d0ddad)
was originally used to fix CPU time going backwards when we `daemonize`.

While, we don't seem to run into this problem on `develop`, I still
think this is a good change to make. We don't need background tasks
running on a process that will soon be forcefully exited and where the
reactor isn't even running yet. We now kick off the background tasks
(`run_as_background_process`) after we have forked the process and
started the reactor.

Also as simple note, we don't need background tasks running in both halves of a fork.

synapse/_scripts/update_synapse_database.py

@@ -120,6 +120,13 @@ def main() -> None:
     # DB.
     hs.setup()
 
+    # This will cause all of the relevant storage classes to be instantiated and call
+    # `register_background_update_handler(...)`,
+    # `register_background_index_update(...)`,
+    # `register_background_validate_constraint(...)`, etc so they are available to use
+    # if we are asked to run those background updates.
+    hs.get_storage_controllers()
+
     if args.run_background_updates:
         run_background_updates(hs)