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