This makes the following changes:
- Adds two new metrics observing the usage of the `DeviceListUpdater`
workers
- Makes the number of workers configurable
- Adds a 30s timeout for DB requests when receiving a device list update
over federation
I hope the comments explain the changes.
`notifyWorkers` notifies a worker which then calls `processServer`,
which in turn gets all users and calls `processServerUser`. There is no
need to call `processServer` for the same domain on startup.
