Add support for elasticsearch 7

[FeLoe]

Elasticsearch 7 is at the moment not supported. In the mapping the 'doc' needs to be removed at the beginning, then the overall mapping works. After that, it still throws an exception related to the client mappings unknown key [mapping.total_fields.limit] for create index that needs to be resolved.

[FeLoe]

And because that makes it more fun: When changing the mapping (without 'doc') it obviously does not work with Elasticsearch 6 anymore. My proposal would be to have two scheme.json files and use one if ES 6 is used and one for ES 7. I'll test around with ES 7 and once it works we can think about doing it like this..