When a user disconnects from the hub, we would spawn off a goroutine
which would make a cluster request, and then update the user status
as offline in the DB.
This was another case of unbounded concurrency where the number of
goroutines spawned was user controlled. Therefore, we would see
a clear spike in DB connections on master when a lot of users
would suddenly disconnect.
To fix this, we implement concurrency control in two areas:
1. In making the cluster request. We implement a counting semaphore
per-hub to avoid making unbounded cluster requests.
2. We use a buffered channel with a periodic flusher to process
status updates.
We also add a new store method to upsert multiple statuses
in a single query. The statusUpdateThreshold is set to 32, which means
no more than 32 rows will be upserted at one time, keeping the
SQL query load reasonable.
https://mattermost.atlassian.net/browse/MM-64298
```release-note
We improve DB connection spikes on user disconnect
by processing status updates in batches.
```
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