Limit the time the scanner can run

Add a max_scanner_duration config value that would indicate the maximum duration the scanne should run. When this value is reached, it should cleanly stop, eventually writing a resume file. When started again, it would read the resume file and try to complete the work again in less than max_scanner_duration else pausing again until it completes the scan.

See #163 (closed) for a justification of such an option.