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