feat: write-through cache

Adds a write-through cache implementation to the `storage` package. The
cache and its operations serve reads from an in-memory map, while writes
are served from Etcd and, if successful, persisted to the in-memory map.
This cache is useful for areas where we need to perform frequent range
queries over small to moderate numbers of small values types. This
implementation is designed to be easy and transparent to add to existing
stores.

Author: jason-lynch