Move device changes off the main process (#18581)

The main goal of this PR is to handle device list changes onto multiple
writers, off the main process, so that we can have logins happening
whilst Synapse is rolling-restarting.

This is quite an intrusive change, so I would advise to review this
commit by commit; I tried to keep the history as clean as possible.

There are a few things to consider:

- the `device_list_key` in stream tokens becomes a
`MultiWriterStreamToken`, which has a few implications in sync and on
the storage layer
- we had a split between `DeviceHandler` and `DeviceWorkerHandler` for
master vs. worker process. I've kept this split, but making it rather
writer vs. non-writer worker, using method overrides for doing
replication calls when needed
- there are a few operations that need to happen on a single worker at a
time. Instead of using cross-worker locks, for now I made them run on
the first writer on the list

---------

Co-authored-by: Eric Eastwood <erice@element.io>

tests/module_api/test_api.py

@@ -28,7 +28,7 @@ from synapse.api.constants import EduTypes, EventTypes
 from synapse.api.errors import NotFoundError
 from synapse.events import EventBase
 from synapse.federation.units import Transaction
-from synapse.handlers.device import DeviceHandler
+from synapse.handlers.device import DeviceWriterHandler
 from synapse.handlers.presence import UserPresenceState
 from synapse.handlers.push_rules import InvalidRuleException
 from synapse.module_api import ModuleApi
@@ -819,7 +819,7 @@ class ModuleApiTestCase(BaseModuleApiTestCase):
 
         # Delete the device.
         device_handler = self.hs.get_device_handler()
-        assert isinstance(device_handler, DeviceHandler)
+        assert isinstance(device_handler, DeviceWriterHandler)
         self.get_success(device_handler.delete_devices(user_id, [device_id]))
 
         # Check that the callback was called and the pushers still existed.