Batch inserter support for labels and schema indexes.

o Creating nodes with labels, adding labels and removing labels
o Listing labels for a node
o Creating schema indexes
o Schema indexes are (re-)populated on batch inserter shutdown
o Support for all the above, except creating new schema indexes,
  in the Batch Database
o Define constants for some error messages to avoid breaking tests
o Updates BatchInsertDocTest
Merge pull request #685 from tinwelint/2.0-batch-inserter-schema

Creating and populating schema indexes in the batch inserter. Merging on good faith that it will give us a green build.