Automate Integrity Checks

[n8maninger]

We've added an integrity check endpoint to manually check the integrity of contracts and their stored sectors, but automating this would be helpful in detecting issues early.

An automated check needs to be very careful of additional IO overhead. We don't want to cause excessive wear on the host's disks or bottlenecks for renters trying to access their data.

• How often should the check run?
• How many sectors should be checked per iteration?
• How to determine which sectors to check? Random? Least recently accessed?

Includes #85