Introduce a functional caching abstraction. The purpose is to provide extensible
API with abstractions that are composable. Includes implementations for basic
cache strategy (current) and an LRU cache (which wraps BasicCache).

A custom caching strategy can be injected via Reactor constructor.

All existing caching behavior remains the same (for now).

We weren't using notion of monoatomically increasing tick in LRU cache because
an OrderedMap does not re-order exising items that are set(), so we depend on
remove()+set(). This means the value of OrderedMap isn't used and simplifies to
OrderedSet.

If you call hit() on an item that's not actually in the LRUCache (edge case),
it's a no-op, so we can just return this.