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>

docker/complement/conf/start_for_complement.sh

@@ -54,7 +54,6 @@ if [[ -n "$SYNAPSE_COMPLEMENT_USE_WORKERS" ]]; then
     export SYNAPSE_WORKER_TYPES="\
       event_persister:2, \
       background_worker, \
-      frontend_proxy, \
       event_creator, \
       user_dir, \
       media_repository, \
@@ -65,6 +64,7 @@ if [[ -n "$SYNAPSE_COMPLEMENT_USE_WORKERS" ]]; then
       client_reader, \
       appservice, \
       pusher, \
+      device_lists:2, \
       stream_writers=account_data+presence+receipts+to_device+typing"
 
   fi