FOR REVIEW: (PUP-3555) Add override-able factory for environment cache entries

NOTE: this isn't quite ready for merge; I need to test it a bit more from Puppet Server, but I wanted to get it pushed up for some initial feedback because I have a few questions/concerns.
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This commit adds a new `cache_entry_factory_class` variable that
allows a caller to override the default implementation of the
logic used to create a cache entry for an environment.

It also abstracts out the method that is used to retrieve the
current system time, so that it can be passed in as a parameter
to the factory constructor to support testing.
I am good with this