Add a function to the overlay cache called UpdateAuditHistory, which
allows us to add online or offline audits to a particular node's audit
history, and get that node's "online score" for the configured tracking
period.
The next step will be to use UpdateAuditHistory from inside
BatchUpdateStats/UpdateStats, so that audit history is actually updated
when nodes get audited, and we can suspend nodes based on their online
score.
Change-Id: I2289105e6961e68e829a987ff756b0e576fab120
