Consider removing SingleValueHydratedProperty and CollectionH.P.

[Tomucha]

... in favour of simple HydratedProperty which technically would be CollectionH.P, so you will always return a collection of keys ... for example Collections.singletonList(key).

Current separation is little bit confusing - I followed the API, created HydratedProperty and nothing happened. My suggested API will be more intuitive.

(Definitelly throw some exception in hydrator, if property isn't neither SingleValueHydratedProperty nor CollectionHP.)