API Endpoint: allow booster to be remotely controller and inspected

[dmorn]

It is important to allow users and other tools to inspect the status of a booster server. In particular, it should be possible to:

• [ ] possibility to toggle "active"/"inactive" state of each source
• [ ] inspect each source's metrics, such as # of downloaded/uploaded bytes, errors, download/upload speed, mean bandwidth, etc..
• [x] retrieve general information about the booster server, such as version, listening ports, "health" status etc
• [ ] possibility to set logging level?

[dmorn]

Metrics will be handled in another issue.