The p/c queries must always be wrapped in a CustomQueryWrappingFilter in fquery parser.
Also made assertions more descriptive when failing.
[Test] Strengthen SimpleIndicesWarmerTests.testEagerLoading.
Disabled query size estimation in percolator, because this is too expensive cpu wise.
Lucene's RamUsageEstimator.sizeOf(Object) is to expensive.
Query size estimation will be enabled when a cheaper way of query size estimation can be found.

Closes #5372
Relates to #5339
Significant_terms aggregation identifies terms that are significant rather than merely popular in a set.
Significance is related to the changes in document frequency observed between everyday use in the corpus and
frequency observed in the result set. The asciidocs include extensive details on the applications of this feature.

Closes #5146
upgrade randomized-testing to 2.1.1

Note that the standard `atLeast` implementation has now Integer.MAX_VALUE as upper bound, thus it behaves differently from what we expect in our tests, as we never expect the upper bound to be that high.
Added our own `atLeast` to `AbstractRandomizedTest` so that it has the expected behaviour with a reasonable upper bound.
See https://github.com/carrotsearch/randomizedtesting/issues/131
[Doc] Add a chart about the relative error of the percentiles aggregation.