A higher level way to use clojure.core.cache that assumes the immutable
cache is wrapped in an atom.

The API is (almost) the same as clojure.core.cache -- including the factory
functions -- but instead of accepting immutable caches, the functions
here accept atoms containing those caches. The factory functions return
new atoms containing the newly created cache.

In addition, lookup-or-miss provides a safe, atomic way to retrieve a
value from a cache or compute it if it is missing, without risking a
cache stampede.