* Removed all hardcoded logic for _{fieldname}_cache.
* Added an internal API for interacting with the field values cache.
Thanks carljm and MarkusH for support.
The only reason why GenericForeignKey and GenericRelation are stored
separately inside _meta is that they need to be cloned for every model
subclass, but that's not true for any other virtual field. Actually,
it's only true for GenericRelation.
The old names were downright confusing. Some seemed to mean the opposite
of what the class actually did.
The new names follow a consistent nomenclature:
(Forward|Reverse)(ManyToOne|OneToOne|ManyToMany)Descriptor.
I mentioned combinations that do not exist in the docstring in order to
help people who would search for them in the code base.
