Make nodesGetFromIndexLookup return PrimitiveLongIterator

o Introduce Neo4jMockitoHelpers.{asAnswer|asPrimitiveAnswer}
o Make IndexReader.lookup return PrimitiveLongIterator
o Introduce JavaConversionSupport for converting PrimitiveLongIterator to scala Iterators
o Delete unused PrimitiveIteratorUtil
o Replace use of HitsIterator with a PrimitiveLongIterator from Hits
Make nodeGetLabels return PrimitiveLongIterator