Background process that checks for corrupted block and failed disks

Currently, CFile corruption and failed disks will result in any bad tablets being marked as failed, being re-replicated elsewhere, and any scans that were in progress for them being retried at other servers.

Rather than waiting for the first bad access to do this, we may want to implement a background task that checks for corruption and proactively re-replicates such tablets. That way, especially when there are long periods of client inactivity, we can the faulty-hardware-related re-replication out of the way.

The task should probably only run when the tserver isn't serving many scans or writes. It should also avoid polluting the block cache, if attempting to check for CFile corruption.

HDFS has a "disk checker" task that may be worth drawing inspiration from.