Core Service: add getPlayerIds() method (#670).

The Core Service provides a new method:
{{{
long[] getPlayerIds(long assocId)
}}}

When an association ID is given it returns the 2 player IDs of that association without
fetching and instantiating a whole `Association` object.

BREAKING CHANGES

The `DeepaMehtaStorage` interface is extended. Storage layer implementations must be adapted.
The Neo4j based implementation is adapted already.

See #670.