wubloader/backfiller
Mike Lang 71333cf826 backfiller: Only run one manager, not one per channel
Then treat backfilling each channel just like backfilling each quality.

This is conceptually simpler (only one kind of thing, a (channel, quality))
and has better behaviour when a node is down (we only have one lot of error handling around it).

It also means we aren't asking the database for the same info once per channel,
and cuts down on logging noise.
..
backfiller backfiller: Only run one manager, not one per channel
Dockerfile Refactor dockerfiles for more shared layers
setup.py Bug fixes to get the database connection working