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>

docs/workers.md

@@ -238,7 +238,8 @@ information.
     ^/_matrix/client/unstable/im.nheko.summary/summary/.*$
     ^/_matrix/client/(r0|v3|unstable)/account/3pid$
     ^/_matrix/client/(r0|v3|unstable)/account/whoami$
-    ^/_matrix/client/(r0|v3|unstable)/devices$
+    ^/_matrix/client/(r0|v3)/delete_devices$
+    ^/_matrix/client/(api/v1|r0|v3|unstable)/devices(/|$)
     ^/_matrix/client/versions$
     ^/_matrix/client/(api/v1|r0|v3|unstable)/voip/turnServer$
     ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/event/
@@ -257,7 +258,9 @@ information.
     ^/_matrix/client/(r0|v3|unstable)/keys/changes$
     ^/_matrix/client/(r0|v3|unstable)/keys/claim$
     ^/_matrix/client/(r0|v3|unstable)/room_keys/
-    ^/_matrix/client/(r0|v3|unstable)/keys/upload$
+    ^/_matrix/client/(r0|v3|unstable)/keys/upload
+    ^/_matrix/client/(api/v1|r0|v3|unstable/keys/device_signing/upload$
+    ^/_matrix/client/(api/v1|r0|v3|unstable)/keys/signatures/upload$
 
     # Registration/login requests
     ^/_matrix/client/(api/v1|r0|v3|unstable)/login$
@@ -282,7 +285,6 @@ Additionally, the following REST endpoints can be handled for GET requests:
 
     ^/_matrix/client/(api/v1|r0|v3|unstable)/pushrules/
     ^/_matrix/client/unstable/org.matrix.msc4140/delayed_events
-    ^/_matrix/client/(api/v1|r0|v3|unstable)/devices/
 
     # Account data requests
     ^/_matrix/client/(r0|v3|unstable)/.*/tags
@@ -329,7 +331,6 @@ set to `true`), the following endpoints can be handled by the worker:
     ^/_synapse/admin/v2/users/[^/]+$
     ^/_synapse/admin/v1/username_available$
     ^/_synapse/admin/v1/users/[^/]+/_allow_cross_signing_replacement_without_uia$
-    # Only the GET method:
     ^/_synapse/admin/v1/users/[^/]+/devices$
 
 Note that a [HTTP listener](usage/configuration/config_documentation.md#listeners)
@@ -550,6 +551,18 @@ the stream writer for the `push_rules` stream:
 
     ^/_matrix/client/(api/v1|r0|v3|unstable)/pushrules/
 
+##### The `device_lists` stream
+
+The `device_lists` stream supports multiple writers. The following endpoints
+can be handled by any worker, but should be routed directly one of the workers
+configured as stream writer for the `device_lists` stream:
+
+    ^/_matrix/client/(r0|v3)/delete_devices$
+    ^/_matrix/client/(api/v1|r0|v3|unstable)/devices/
+    ^/_matrix/client/(r0|v3|unstable)/keys/upload
+    ^/_matrix/client/(api/v1|r0|v3|unstable/keys/device_signing/upload$
+    ^/_matrix/client/(api/v1|r0|v3|unstable)/keys/signatures/upload$
+
 #### Restrict outbound federation traffic to a specific set of workers
 
 The