Fixed a bug in date_histogram aggregation parsing

 - pre_zone_adjust_large_interval was not parsed properly
 - added tests for pre_zone and pre_zone_adjust_large_interval
 - changed DateHistogram#getBucketByKey(String) to support date formats (next to numeric strings)
 - added randomized testing for fetching the bucket by key in date_histogram tests
 - added missing "format" support in DateHistogramBuilder

 Closes #5375
Introduced VersionType.FORCE & VersionType.EXTERNAL_GTE

Also added "external_gt" as an alias name for VersionType.EXTERNAL , accessible for the rest layer.

Closes #4213 , Closes #2946
REST Testing framework enhancement

Adding operators 'lte' and 'gte' to our REST test framework. These
operators test for, respectively, less-than-or-equal and
greater-than-or-equal.
Merge pull request #5360 from aleph-zero/issue-111

REST Testing framework enhancement