Completes the IdMapper interface change

by letting InputRelationship deal with Object ids for start/end nodes.
Now it became apparent that stages 1 and 2 cannot be executed in parallel
if the IdMapper needs preparation. Put that check in and added comments
about it.
Merge branch '2.1-maint'

Conflicts:
	community/kernel/src/main/java/org/neo4j/unsafe/impl/batchimport/RelationshipEncoderStep.java
	community/neo4j/src/test/java/upgrade/StoreUpgraderTest.java