Index with label constraint does not work for partitioned labels

Code to reproduce:

```
graph = JanusGraphFactory.open("inmemory")
mgmt = graph.openManagement()
l = mgmt.makeVertexLabel('V').partition().make()
p1 = mgmt.makePropertyKey('p').cardinality(Cardinality.SINGLE).dataType(String.class).make()
mgmt.buildIndex("i1", Vertex.class).addKey(p1).indexOnly(l).buildCompositeIndex()
mgmt.commit()
g = graph.traversal()

v1=g.addV("V").property('p','a').next()
g.tx().commit()
g.V().hasLabel('V').has('p', 'a')
```
```
gremlin> mgmt = graph.openManagement()
==>org.janusgraph.graphdb.database.management.ManagementSystem@277bc3a5
gremlin> l = mgmt.makeVertexLabel('V').partition().make()
==>V
gremlin> p1 = mgmt.makePropertyKey('p').cardinality(Cardinality.SINGLE).dataType(String.class).make()
==>p
gremlin> mgmt.buildIndex("i1", Vertex.class).addKey(p1).indexOnly(l).buildCompositeIndex()
==>i1
gremlin> mgmt.commit()
==>null
gremlin> g = graph.traversal()
==>graphtraversalsource[standardjanusgraph[inmemory:[127.0.0.1]], standard]
gremlin>
gremlin> v1=g.addV("V").property('p','a').next()
==>v[4226]
gremlin> g.tx().commit()
==>null
gremlin> g.V().hasLabel('V').has('p', 'a')
gremlin>
```
Related issue #206

Start by running the provided in-memory reproduction using a partitioned vertex label, a composite index with indexOnly, and the g.V().hasLabel('V').has('p', 'a') traversal. Trace composite-index handling for partitioned labels and add a regression test for this case. Done means the traversal finds the committed vertex through the index.

Written by the indexing model from the issue text.