[TESTS] Ensure yellow in SimpleFacetsTests.testFilterFacetWithFacetFilterPostMode.
Aggregations: Added pre and post offset to histogram aggregation

Added preOffset and postOffset parameters to the API for the histogram aggregation which work in the same way as in the date histogram

Closes #6605
Add `wait_if_ongoing` option to _flush requests

This commit adds the ability to force blocking on the flush operaition
to make sure all files have been written and synced to disk. Without
this option a flush might be executing at the same time causing the
current flush to fail and return before all files being synced.

Closes #6996