Add hostdb endpoint to indicate intial scan status and queued scans

[ChrisSchinnerl]

This Pr adds a new endpoint that lets the user retrieve information about the hostdb. At this point it contains the list of hosts that are being scanned at the moment and the a flag indicating if the initial scan of the hostdb is complete. That is useful since we currently block uploads and contract formation/renewals until the initial scan is done. Once this endpoint is reviewed and after I've received some feedback I'll update the documentation and add an automated test for it.

[DavidVorick]

Still need a test demonstrating that the endpoint is working. Or need to extend an existing hostdb test to include using this endpoint to verify that the scanning stuff is working correctly.