Reduce churn of cache on revocation_list

The explicit call to refresh will cause higher-than-average load
under high counts of revocation events.  The simple invalidate ensures
that we don't re-generate the revocation list on revocation event
but only on-demand.

closes-bug: 1220359
Change-Id: I826778843e344be001bbcc832054a0ad446285e7
Merge "Reduce churn of cache on revocation_list"