Add support of ElasticSearch 6

[To-om]

The migration from ES5 to ES6 follows the recommendations from https://www.elastic.co/guide/en/elasticsearch/reference/6.0/removal-of-types.html. Document types are merged in "doc" document. Type information is stored in the join field, named "relations".

API doesn't change but the migration of data is needed. Data doesn't need modification except for sequences. Migration operations look like:

Seq(
  mapEntity("sequence") { seq =>
    val oldId = (seq \ "_id").as[String]
    val counter = (seq \ "counter").as[JsNumber]
    seq - "counter" - "_routing" +
      ("_id" -> JsString("sequence_" + oldId)) +
      ("sequenceCounter" -> counter)
  }
)

The migration process needs also to know if the index uses mapping type or not. This is done by implementing the MigrationOperations.indexType method.