PersistenceUnits#instance singleton does not work in EAR scenarios

Currently the info regarding the various persistence.xml files on the classpath only gets scanned once and stored in the static PersistenceUnits#instance.
This does not work in EAR scenarios if one of the persistence.xml is in a webapp or if the deltaspike-data-impl is on any other shared ClassLoader.

We should at least document this restriction it until it's properly fixed.

A possible fix would be to have a (weak) Map<ClassLoader, PersistenceInfo> which also looks up the parent ClassLodaer chain.