Add endpoint for usable hosts to bus

[ChrisSchinnerl]

The bus should have an endpoint that returns a list of good hosts. That includes the public key of the host and its siamux address.

The hosts should be

• good according to the autopilot
• not blocked
• not gouging
  at the time they are returned from the bus

The goal of the PR should be to

1. introduce that new endpoint
2. change DownloadContracts in the cache to UsableHosts, making use of the new endpoint while also making sure the download code as well as the account refill code don't rely on contracts anymore.

Ideally this leads to only the upload code still requiring contracts and gouging checks in the worker.