Fix trigger cache maintenance

Each pipeline was maintaining the trigger cache only based on
its own current usage, which means that pipelines were clearing
the cache of changes currently being used by other pipelines.

This considers all changes in use in all pipelines when maintaining
the cache.

Change-Id: I3ab14c69acd80ecc613b63628c837511594744d0
Reviewed-on: https://review.openstack.org/36699
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins