When re-syncing device lists reset the state

We keep track of what stream IDs we've seen so that we know what updates
we've handled or missed. If we re-sync we don't know if the updates
we've seen are included in the re-sync (there may be a race), so we
should reset the seen updates.

synapse/handlers/device.py

@@ -566,6 +566,10 @@ class DeviceListEduUpdater(object):
                 )
                 device_ids = [device["device_id"] for device in devices]
                 yield self.device_handler.notify_device_update(user_id, device_ids)
+
+                # We clobber the seen updates since we've re-synced from a given
+                # point.
+                self._seen_updates[user_id] = set([stream_id])
             else:
                 # Simply update the single device, since we know that is the only
                 # change (because of the single prev_id matching the current cache)