Make GraphServer use PartitionGraph

Continue with adding support for multi tenancy.
Investigated making a wrapper interface for both KeyIndexableGraph and
IndexableGraph to support the ID/Partition combination and avoid the
hidden gotcha at line 57, but the added complexity of the code wasn't
worth it.  These interfaces are going away in TinkerPop 3 anyway, see
https://groups.google.com/forum/#!topic/gremlin-users/Op2QZFdJ15A