Races in device list update logic

[richvdh]

1. at sync token 1, we get a notification that alice has updated her devices, and start a request to update them
2. at sync token 2, we get a notification that bob has updated his devices, and start a request to update them
3. the request for bob's devices completes, so we consider ourselves to be up-to-date to token 2.
4. we restart the app
5. we have an outdated list of alice's devices, but no way to tell that.

[richvdh]

The easy solution here is to limit ourselves to one update request in flight.

[richvdh]

Another one:

1. After initial sync (sync token 1, say), but before the /changes query completes, the user initiates a device query for Bob (eg by looking at their memberinfo).
2. That completes; we consider ourselves to be up-to-date to token 1.
3. Still before /changes completes, or before the resulting /query completes, we restart the app
4. We may have many outdated device lists, but no way to tell that.