Support separate graph and job configs in ScanJob

This was motivated by IndexRemoveJob, which implements ScanJob but
wants both a config rooted at GraphDatabaseConfiguration.JOB_NS and
another config rooted at GraphDatabaseConfiguration.ROOT_NS.  This may
not be the best way to do it, but it fixes IndexRemoveJob on
non-inmemory executors.
Add index maintenance helper methods

The MapReduceIndexJob class contains static methods for one-liner
index remove/repair operations, similar to TitanIndexRepair in the
titan05 tree