[Persistence] Use StepBehindCache for bootstraps

This consolidates code around solutions for
nasa/mct#111
[Persistence] Test step-behind cache
[Persistence] Pre-initialize all-user cache

Load the all-user cache when the Persistence
Provider is first activated. This ensures
that when some user action needs to make use
of this cache it has some value to return;
otherwise, an actual database lookup will be
required to complete the request, which can
be time-consuming.