Optimize label retrieval

Any time when [AbstractVertex](https://github.com/JanusGraph/janusgraph/blob/master/janusgraph-core/src/main/java/org/janusgraph/graphdb/vertices/AbstractVertex.java#L125)
.label() method is to invoked it lead to expensive query [constructQueryWithoutProfile](https://github.com/JanusGraph/janusgraph/blob/master/janusgraph-core/src/main/java/org/janusgraph/graphdb/query/vertex/BasicVertexCentricQueryBuilder.java#L417)

But this type of query may be evaluted one time and reused

Start in janusgraph-core/src/main/java/org/janusgraph/graphdb/vertices/AbstractVertex.java at label(), then inspect constructQueryWithoutProfile in janusgraph-core/src/main/java/org/janusgraph/graphdb/query/vertex/BasicVertexCentricQueryBuilder.java. Determine how the expensive label query is constructed and whether its result can be evaluated once and reused. Done means repeated label() calls avoid redundant query evaluation without changing label behavior.

Written by the indexing model from the issue text.