Fix CPU time going backwards when daemonize

When we `daemonize`, we fork the process and cputime metrics get confused
about the per-thread resource usage appearing to go backwards because we're
comparing the resource usage (`rusage`) from the original process to the
forked process.

We now kick off the background tasks (`run_as_background_process`) after we
have forked the process so the `rusage` we record when we `start` is in the
same thread when we `stop`.

Bad log examples from before:
```
synapse.logging.context - ERROR - _schedule_next_expiry-0 - utime went backwards! 0.050467 < 0.886526
synapse.logging.context - ERROR - _schedule_db_events-0 - stime went backwards! 0.009941 < 0.155106
synapse.logging.context - ERROR - wake_destinations_needing_catchup-0 - stime went backwards! 0.010175 < 0.130923
synapse.logging.context - ERROR - resume_sync_partial_state_room-0 - utime went backwards! 0.052898 < 0.886526
```

Testing strategy:

 1. Run with `daemonize: true` in your `homeserver.yaml`
 1. `poetry run synapse_homeserver --config-path homeserver.yaml`
 1. Shutdown the server
 1. Look for any bad log entries in your homeserver logs:
    - `Expected logging context sentinel but found main`
    - `Expected logging context main was lost`
    - `utime went backwards!`/`stime went backwards!`

synapse/app/_base.py

@@ -601,6 +601,12 @@ async def start(hs: "HomeServer") -> None:
     hs.get_datastores().main.db_pool.start_profiling()
     hs.get_pusherpool().start()
 
+    # Register background tasks required by this server. This must be done
+    # somewhat manually due to the background tasks not being registered
+    # unless handlers are instantiated.
+    if hs.config.worker.run_background_tasks:
+        hs.start_background_tasks()
+
     # Log when we start the shut down process.
     hs.get_reactor().addSystemEventTrigger(
         "before", "shutdown", logger.info, "Shutting down..."

synapse/server.py

@@ -366,12 +366,6 @@ class HomeServer(metaclass=abc.ABCMeta):
         self.datastores = Databases(self.DATASTORE_CLASS, self)
         logger.info("Finished setting up.")
 
-        # Register background tasks required by this server. This must be done
-        # somewhat manually due to the background tasks not being registered
-        # unless handlers are instantiated.
-        if self.config.worker.run_background_tasks:
-            self.setup_background_tasks()
-
     def __del__(self) -> None:
         """
         Called when an the homeserver is garbage collected.
@@ -410,7 +404,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         appropriate listeners.
         """
 
-    def setup_background_tasks(self) -> None:
+    def start_background_tasks(self) -> None:
         """
         Some handlers have side effects on instantiation (like registering
         background updates). This function causes them to be fetched, and