store: batching support

[shuttie]

Some events are triggering too many random writes from db. It can be OK if there is proper caching (narrator: it is not yet), but in some cases we need the way to pull multiple values at the same time.

Main use case: the reranking request. We need to pull all product metadata at once, otherwise the latency can be extremely high.

As this feature is not critically important and not affecting customer-facing features, then it can be optional.

[shuttie]

done