Auto-Renew + offline checks

The repair loop will now automatically renew uploaded files. For now, all user-uploaded files are auto-renewed; we'll have to add a call to enable/disable this behavior. (Loaded .sia files are not renewed, since they are not in the renter's repair set.) Contracts are renewed when they are less than 2000 blocks away, and are renewed for 6000 blocks in the future.

Hosts are also checked for availability, and their contracts will be replaced if they do not respond to a ping within 15 seconds. This check is both slow and heavy-handed, so we may want to consider leaving it disabled until it is more sophisticated.