Fix computation of explanations for AllTermQuery.

The use of freq() instead of sloppyFreq() and the fact that `numMatches` was
not updated in `setFreqCurrentDoc` could lead to an inaccurate score in the
explanation.

Close #4298
Changed the minScore comparator from > to >=

Closes #4303
Added simple test for minScore
trimmed down SimpleQueryTests + more assertAcked
NullPointerException in FsTranslog when reverting transient log
This happens when reverting the trans transaction log on failure, and when that happens, actually we might have failed on the transient translog creation to being with....
fixes #4223
added assertions for shard failures in stats agg tests
added more assertions to aggs tests
Cat API: Improve help and execute help before sending server requests

* Force the cat API classes to have simple help available by extending from a AbstractCatAction
* Use a set binder on guice creation to create the help on node start up
* Make sure that the help/field info is returned without querying any data
wait for green in tests for now
Fixes related to accepted docs not taken into account:
* Removed the applyAcceptedDocs in ChildrenConstantScoreQuery, they need to be applied at all times. (because of short circuit mechanism)
* Moved ParentDocSet to FilteredDocIdSetIterator, because it fits better than MatchDocIdSet.
* Made similar changes to ParentConstantScoreQuery for consistency between the two queries. The bug accepted docs bug didn't occur in the ParentConstantScoreQuery.
* Updated random p/c tests to randomly update parent or child docs during the test run.

Closes #4306
Also short circuit within segment when score mode is avg
Make the ChildrenQuery and ParentQuery not rely on the fact that score of 0 means that there isn't a match.
add more ensureGreen to aggs tests
Log intermediate pending searchers as trace instead of debug
The short circuit mechanism always needs to be wrapped with the live docs. In certain scenarios the live docs isn't passed down with acceptedDocs.
For example when a has_child is wrapped in a filtered query as query and the wrapped filter is cached.
The short circuit mechanism in that case counts down based on deleted docs, which then yields lower results than is expected.

Relates to #4306
Added random filtered query logic the to random parent query tests.
Fixed bug in ParentQuery.ChildScorer#advance(), the parent value was checked for the _uid field while the _parent should have been checked.
Changed the "script_lang" parameter to "lang" in all value source based aggs - to be consistent with all other script based APIs.
[DOCS] Fixed typos and corrected grammar
only wrap the parentFilter in ApplyAcceptedDocsFilter, not the other short circuit filters.
fix mapping error in p/c benchmarks