IdMapping interface for BatchImporter that makes sense

derived from actual use case. Although there's only one implementation,
which is to use the actual node ids provided directly from the input
entities.

Basically the changes consist of:
* having InputEntity#id(), InputRelationship#startNode()/endNode() return Object
* IdMapper has something closer to an index interface with put and get
* IdMapping is provided to BatchImporter#doImport and couple an IdMapper
  and IdGenerator (batch import version of an id generator)
* NodeStage uses IdMapper#put in conjunction with IdGenerator#generate and
  IdMapper#sort in the end
* RelationshipStage calls IdMapper#get