Merge lightning history with the live feed

The time to the millisecond and the position to four decimals are the identity the MQTT feed uses, so a flash heard live and the same flash read from GET /lightning/flashes match exactly.

1. Load the history from the REST route.
2. Key a set on t_ms,lat,lon and put every historical flash in it.
3. Subscribe to the MQTT lightning feed and drop live messages already in the set.
4. Fade flashes on t_ms, not on when you received them.