Fix bug where device lists would break sync (#17292)

If the stream ID in the unconverted table is ahead of the device lists
ID gen, then it can break all /sync requests that had an ID from ahead
of the table.

The fix is to make sure we add the unconverted table to the list of
tables we check at start up.

Broke in https://github.com/element-hq/synapse/pull/17229