Improve detection of checksum changes on pillars

A crucial ingredient in efficient integrity checking is how updated the integrity database is. We should therefore focus on maintaining a up-to-date representation in the integrity service DB.

This could be improved by either:

• Frequently getting all checksum every time.
• Frequently requesting checksums which have been updated since 'time-for-the-last-check'. The prefered one, requires a minimal protocol update to allow a checksumsNewerThan(time) request.