Add a new option, workers, that works more or less like the same option
from background daemons. Disks will be distributed across N worker
sub-processes so we can make the best use of the I/O available.
While we're at it, log final stats at warning if there were errors.
Co-Authored-By: Clay Gerrard <clay.gerrard@gmail.com>
Change-Id: I039d2b8861f69a64bd9d2cdf68f1f534c236b2ba
abfa6bee72