Make sure database metric values are up to date

[fergusL]

Currently the screener service will calculate metrics for any file that fails the screen_success() check. When we add a new metric, the screen_success() will also be updated to check the new metric, if an entry does not have the new metric it will fail the screen success check and have a new set of metrics calculated.

If a metric function gets updated, there needs to be a way to determine that the entries in the database contain old metric values and need to be recalculated. One way of doing this is to include the date of screening with each metric value, which will then be used somehow by screen_success to determine if the metric was calculated with the most recent version of the metric... so somehow need to record when metric functions were last updated?