CompositeIndex‘s unique fail

**description: First, I create unique CompositeIndex. Then, Two terminas execute g.addV().property("name", "user2"). It's success. It's should be fail.**

- schema
```
graph = JanusGraphFactory.open("conf/graph1.properties")
m = graph.openManagement();
m.makePropertyKey('name').dataType(String.class).make()
m.commit()

m = graph.openManagement();
m.buildIndex('idx1_name', Vertex.class).addKey(m.getPropertyKey('name')).unique().buildCompositeIndex()
m.commit()
graph.tx().commit()

```
- termina1
```
gremlin> graph = JanusGraphFactory.open("conf/graph1.properties")
==>standardjanusgraph[hbase:[127.0.0.1]]
gremlin> g=graph.traversal()
==>graphtraversalsource[standardjanusgraph[hbase:[127.0.0.1]], standard]

gremlin> g.addV().property("name", "user2")
==>v[8192]
gremlin> g.tx().commit()
==>null
gremlin> g.V().valueMap(true)
==>[id:4096,name:[user3],label:user]
==>[id:8192,name:[user2],label:vertex]
==>[id:4112,label:vertex]
==>[id:8208,name:[user1],label:vertex]
==>[id:4192,name:[user3],label:user]
==>[id:4304,name:[user2],label:vertex]

```

- termina2
```
gremlin> graph = JanusGraphFactory.open("conf/graph1.properties")
==>standardjanusgraph[hbase:[127.0.0.1]]
gremlin> g=graph.traversal()
==>graphtraversalsource[standardjanusgraph[hbase:[127.0.0.1]], standard]

gremlin> g.addV().property("name", "user2")
==>v[4304]
gremlin> g.tx().commit()
==>null
gremlin> g.V().valueMap(true)
==>[label:user,name:[user3],id:4096]
==>[label:vertex,name:[user2],id:8192]
==>[label:vertex,id:4112]
==>[label:vertex,name:[user1],id:8208]
==>[label:user,name:[user3],id:4192]
==>[label:vertex,name:[user2],id:4304]

gremlin> g.addV().property("name", "user2")
Adding this property for key [name] and value [user2] violates a uniqueness constraint [idx1_name]
Type ':help' or ':h' for help.
Display stack trace? [yN]
```

Reproduce the behavior from the issue using graph.openManagement(), buildIndex(...).unique().buildCompositeIndex(), and two Gremlin sessions running g.addV().property("name", "user2") followed by commits. Trace how the composite index enforces uniqueness across those transactions; done means duplicate values are rejected consistently rather than committed by both terminals.

Written by the indexing model from the issue text.