Add more tests, factorize more code

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Move equivalences around, create new ProcessingCache class

We now have two equivalences, group them in the same package.

Also create a ProcessingCache class, which is in essence a LoadingCache whose
keys are wrapped in an Equivalence.Wrapper.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>