Speed up consistency checking of non-constraint indexes

This improves the time complexity for consistency checking of normal non-constraint schema
indexes from an O(n^2) to O(n).

This is especielly noticable when many nodes are indexed by the same
<label,propertyKey,propertyValue> tuples.

The old code would go over all the nodes that were supposed to be indexed, and then for
each of those nodes, do a lookup on the propertyValue, and then iterate through the result
set of that look up, looking for the nodeId in question. This is obviously an oh-n-squared
amount of work, if all the nodes are indexed with the same property value.

The new code do an exact lookup for the combined <nodeId,propertyValue> in the given
<label,propertyKey> index. For our purpose, we consider this a constant cost, and so our
work now ends up being oh-n.
Merge pull request #2520 from chrisvest/2.1-consistency-check-indexes

Speed up consistency checking of non-constraint indexes