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
Fixes a license header
Merge branch '2.1-maint'
Evict from cache as late as possible in race test.
Exact number of relationships given node label.

The CountsStore maintains the exact number of relationships in the
database that either starts or ends in a node with a given label. Counts
are maintained both in total, and by relationship type, but only for one
label, i.e. number of relationships that starts in a node with a given
label and ends in a node with a given label is not maintained.