Refactor getter caching based on keypath state

[jordangarcia]

The current version of NuclearJS uses a cache key consisting of store
states (monotomically incresing ID per store). This has the
disadvantage of allowing only a single level of depth when figuring out
if a cache entry is stale. This leads to poor performance when the
shape of a Reactor's state is more deep than wide, ie a store having
multiple responsibilities for state tracking.

The implementation is as follows:

• Consumer can set the maxCacheDepth when instantiating a reactor
• Getters are broken down into the canonical set of keypaths based on
  the maxCacheDepth
• Add a keypath tracker abstraction to maintain the state value of all
  tracked keypaths
• After any state change (