Integrate foundationdb plugin into janusgraph main repository

Hi folks, I am wondering why we don't integrate the foundationdb adapter into janusgraph/janusgraph main repository. I have never involved in the design/implementation of the janusgraph-foundationdb adapter, so my view might be one-sided, but from what I understand, this seems to be due to a historical reason: the foundationDB was transferred from experoinc to janusgraph namespace. While it totally makes sense to make the plugin a separate repository when it is maintained by a third party (like other adapters), I don't see a specific benefit of keeping it as a separate project under the janusgraph namespace.

Benefits of integration:
1) New changes can be released together with the janusgraph main project. This typically is a drawback because a standalone project usually means faster iteration & release. However, from what I have observed, there has been no new release of the foundationdb adapter since Sep 15, 2018.
2) As an ACID-compliant distributed database, integration of foundationdb adapter could possibly improve janusgraph main project. For example, some tests might only be possible to run with foundationdb.
3) More visibility for the foundationdb plugin.
4) Easier to sync since there is no need to wait for main project release. Currently, bug fix / new feature of the main project could block the development of the foundationdb plugin.

Drawbacks of integration:
1) Make the main repository fatter.

Start by reviewing the integration proposal and the existing janusgraph-foundationdb and janusgraph repositories, including the adapter's release history. Determine the repository, release, testing, and maintenance changes required, then document a concrete integration plan or decision in the issue discussion.

Written by the indexing model from the issue text.