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