Multiple sources

Being able to use multiple ADSB or AIS sources would be great.

That adds another issue : deduplication.

The needed refactoring is:
- `airwaves_*_client.py` pushes to a redis queue 'collector'
- a daemon will run and listen to 'collector', do some dedup, and then push them to 'state' queue
- backend will then uses 'state' queue instead of 'default' one