rather than force a cache.cache() interface and a uniform expiry across
the cache, we add a more common per store() expiry that can also be used
instead or in addition to the original cache-wide expiry.

previously 'bank' was minions/$minion, and key was 'data', which was a
merge of both pillar and grains. rather than do that, we organize pillar
and grains into separate banks for better index optimizations for
backends that can take advantage of it. it also just makes more sense
this way

took the liberty of making it a proper subclass in the process. this now
uses the salt.cache infrastructure such that it can be driven by the
base cache driver or a different one if so desired. functionality should
be equivalent, including using the base bank=pillar key=minion_id for
merged pillar, such that minion_data_cache can take advantage of the
same cache. because we are updating the cache at the source, we no
longer need to update the cache in master/masterapi.

the loops involved dont use sets which seems to have an oversized impact
on lookup performance. using sets for containment checks makes this
function go 15x faster on a 50k dataset (from 30+ sec to 2~ sec).