Clean up ShardedWorkerHandlingConfig (#9466)

* Split ShardedWorkerHandlingConfig

This is so that we have a type level understanding of when it is safe to
call `get_instance(..)` (as opposed to `should_handle(..)`).

* Remove special cases in ShardedWorkerHandlingConfig.

`ShardedWorkerHandlingConfig` tried to handle the various different ways
it was possible to configure federation senders and pushers. This led to
special cases that weren't hit during testing.

To fix this the handling of the different cases is moved from there and
`generic_worker` into the worker config class. This allows us to have
the logic in one place and allows the rest of the code to ignore the
different cases.

synapse/push/pusherpool.py

@@ -59,7 +59,6 @@ class PusherPool:
     def __init__(self, hs: "HomeServer"):
         self.hs = hs
         self.pusher_factory = PusherFactory(hs)
-        self._should_start_pushers = hs.config.start_pushers
         self.store = self.hs.get_datastore()
         self.clock = self.hs.get_clock()
 
@@ -68,6 +67,9 @@ class PusherPool:
         # We shard the handling of push notifications by user ID.
         self._pusher_shard_config = hs.config.push.pusher_shard_config
         self._instance_name = hs.get_instance_name()
+        self._should_start_pushers = (
+            self._instance_name in self._pusher_shard_config.instances
+        )
 
         # We can only delete pushers on master.
         self._remove_pusher_client = None