Add Couchbase backend

**Describe the feature:**
We at couchbase developed a backend for JanusGraph that runs on top of a Couchbase cluster and decided to share it with the community.

The backend uses Couchbase cluster's key-value storage to store graph data and FTS service for searches.

**Describe a specific use case for the feature:**
Running JanusGraph on an existing Couchbase cluster.
Launching JanusGraph on a single database without the need to configure additional search engines like Solr (which is a great product, of course, but not everyone has the know-how to work with it).

No files, tests, or entry points are named. Start by reviewing JanusGraph's existing storage and search backend abstractions, then determine the scope for integrating Couchbase key-value storage and FTS; done would mean a supported backend that runs JanusGraph on a Couchbase cluster without an additional search engine.

Written by the indexing model from the issue text.