Direct index query has no option to return just the vertex or edge ids

A Solr indexing backend was used for this test. On a Solr search, one has the option to specify the properties returned from the search. Returning just a list of ids as opposed to the entire vertex yields better performance and also allows us to traverse further on the result set like:
`g.V(solrIds).has(...)`

Currently there is `JanusGraphIndexQuery.vertexStream()`, it would be nice to have a similar function for returning the ids such as `JanusGraphIndexQuery.vertexIds()` which returns a stream or list of strings which are vertex IDs that can be passed into a gremlin query.

Start by tracing JanusGraphIndexQuery.vertexStream() and the Solr search path mentioned in the issue. Define the corresponding vertexIds() entry point so it returns ids that can be supplied to g.V(solrIds), and verify that the result supports the intended follow-up traversal.

Written by the indexing model from the issue text.