Provide method to limit invalidation of specific models for specific periods of time.

[mikebronner]

Continuation of discussion from #59.

My thoughts on this are the following:

• developer provides a protected property on a model that defines the minimum period for which its cache will not be invalidated.
• invalidation mechanism will check for existence of that property, and compare it against a cache value that identifies the model and the timestamp of last invalidation.
• invalidation will proceed if sufficient time has passed as defined by the protected property on the model.
• after invalidation, the cache value for that model will be reset with a new timestamp.

If a model does not contain the protected property, none of the above will take place, and cache invalidation will proceed as normal.

[mikebronner]

Working on this feature in a livestream. Feel free to join me here: https://www.larastream.com/r/d596ec7