* Re-create PR #4527 from @arcolife:
fixes #4526 - add nested support to fieldname autocomplete; Also:
    - update _mapping to first use given time range
      for deducting index mapping, then fallback to
      today's date based index name

* (elasticsearch): refactor getFields() method.

* (elasticsearch): add tests for getFields() method.

* (elasticsearch): fixed _get() method (tests was broken after @arcolife commit).