device_lists_stream table is large and gets queried a lot

[erikjohnston]

device_lists_stream is one of the busiest tables in terms of number of rows fetched from it. This turns out to be because clients can ask us to do an unbounded fetch for all devices that have changed between two tokens, which returns a lot if the tokens are old.

We also store a row per device, but we don't actually seem to read the device.