Do not use JanusGraphFactory.drop(graph) with Elasticsearch 6.x

sjudeng <sju...@...>

Anyone running JanusGraph with Elasticsearch 6.x should not make calls to index.clearStorage(), including JanusGraphFactory.drop(graph) and JanusGraphCleanup.clear(graph).

A community member discovered and reported a significant bug in the Elasticsearch clearStorage implementation that causes all aliased indices to be deleted when using Elasticsearch 6.x with index aliases. A fix has been submitted but currently this affects JanusGraph 0.2.0, 0.2.1-SNAPSHOT and 0.3.0-SNAPSHOT.