Rework `Key` vs `Path` nomenclature in the `Store` domain

[plafer]

Currently, in the context of Stores, we use Path and we use Key (here, K was chosen as a short for Key) both the signify the key to a value in the Store.

We either stick with one or the other, or clearly delineate where one should be used instead of the other (e.g. @hu55a1n1 suggested that key should be used at lower levels of abstraction as Path).