The multiGet uses the "for" syntax internally to chain a bunch of computations from T => Future[U].
Compound BatchStore and Mergeable, used for building a summingbird job.
MergeableStore that triggers a side effect on every call to put or merge.
Summingbird ClientStore -- merges offline and online.
Returns a MergeableStore that augments a Summingbird "online store" with the ability to snapshot keys from the combined online/offline ClientStore into some third presenting store every time a key is touched.