Caching mechanism

[Chalarangelo]

Add a caching mechanism to records as follows:

• Expose a readonly Map type named propertyCache.
• Allow for clear() calls on the propertyCache or expose a clearPropertyCache() method.
• Specify on the model which properties are cached (e.g via cachedProperties - array of property names).
• Invalidate cache completely whenever any field on the model changes.
• Disallow (if possible) property caching for relationship reverses.

[Chalarangelo]

Partially implemented in v1.3.0 by providing a cacheProperties on the model and invalidating caches on field changes. Relationship property caching is not allowed.

Future iterations should introduce a clear method for both records and full models.

The readonly propertyCache Map has been implemented as an internal detail for the RecordHandler.

[Chalarangelo]

WeakMaps could also be leveraged for this.

[Chalarangelo]

Depending on #27, we might change this mechanism to an internal and handle it ourselves.