Create JanusGraphIndex::describe method to include more information

[li-boxuan]

Currently, JanusGraphIndexWrapper::toString simply returns the index name. We could create a new describe method (or any other name) so that user could more easily retrieve information about that index. For example, we could include the status of each field key. Currently, it is a bit cumbersome to retrieve such information (see below):

gremlin> mgmt.getGraphIndex("mixed_demo")
==>mixed_demo
gremlin> mgmt.getGraphIndex("mixed_demo").getFieldKeys()
==>name
==>update_time
gremlin> mgmt.getGraphIndex("mixed_demo").getParametersFor(mgmt.getPropertyKey("name"))
==>mapping->STRING
==>mapped-name->name
==>status->ENABLED