Add 'purge' command to clean out old archived and failed items

[pbeckingham]

Some kind of 'central purge' command should clean out old archive entries (older than x) and failed items. If not purged, these otherwise accumulate, although they are small, there are many.

For now there is a script in /opt/flod2/central on sitka that does this:

find central/*/archive -type f -mtime +30 -exec rm {} \;