make index backends pluggable

Is there a way to make index backends pluggable?

For example I created a graph with cassandra + elasticsearch and it turns out I'm not gonna use elasticsearch for indices. So I decided to disable it in JanusGraph configuration and restart the server. After that it gave me an error that it can't connect to elasticsearch.

It would be good if we could disable and enable them at will. And JanusGraph will just work either with indices from index backends or without them without giving errors.

The issue names no files, tests, or entry points. Start by reproducing the restart with Elasticsearch disabled after configuring Cassandra and Elasticsearch, then trace the configuration and startup handling; done should mean index backends can be enabled or disabled without an unwanted connection error.

Written by the indexing model from the issue text.