WIP: Update hostdb to scan hosts with contracts every time (On Hold)

MERGE REQUEST

Changes to Work Flow

Before threadedScan looped over the hosts from the hosttree and built online and offline lists until it had reached hostCheckupQuantity. This method gave no guarantee as to which hosts were getting scanned. I added a contractHosts field to the hostdb and added a method to the contractor and hostdb to update it at the end of threadedContractMaintenance to ensure that it has the most up to date list of hosts that have active contracts with the renter. Now threadedScan starts with the list of contractHosts and then continues on with additional hosts from the hosttree. This ensures that the active hosts with contracts are always scanned so that they are up to date.

Testing Updates

No testing added

Modules and Packages Impacted

renter, contractor, hostdb

closes #3215 (closed), #3042 (closed)