Add support for Elasticsearch 6.0+

[jciolek]

Summary

Due to the breaking change in Elasticsearch 6.0, namely removal of mapping types and consequently changes in parent-child relationship, parent-child handling had to be updated.

Even though in Elasticsearch 6.0 it is a breaking change, I managed to keep the service API consistent, and continue support across the whole range of versions (2.4 - 6.3). The upgrade should not cause any problems for existing installations.

A few important notes:

• New service options have been introduced:
  • esVersion
  • join
  • routing
• Test db schema have been changed slightly in order to accommodate the changes in Elasticsearch described above. Consequently, the tests have been changed as well, in order to avoid unnecessary repetition.
• The core functions are now gathered in a core container, which became a property of the service. The container is created for each service instance and selects appropriate versions of the core functions, based on the esVersion options. This ensures that dependencies between core functions are always valid.
• I have also replaced dated istanbul with nyc.