Try to make faster ToParentBlockJoinQuery.explain()

[mkhludnev]

Description

It's a spin off from #12204 #12245.
I'm not sure it's really necessary, please advise.
Now ToParentBlockJoinQuery.BlockJoinScorer#explain requests expensive explain() for every matching child doc. Here are possible measures:
2. call score() in the beginning of explain(), use returned score to find child doc (for Min,Max modes)
3. drag child scorer over block to count matches
4. request explain() only for the child found at 2.

WDYD?

[MarcusSorealheis]

it can be implemented, but how much effort into explain performance at this time. I don't think now is the time to address this aspect of a feature that has been broken for years and remains broken.

[MarcusSorealheis]

it's a great idea, though. There's likely higher priority work, even in ToParentBlockJoinQuery.

[mkhludnev]

also, worth to fix 963ed7c#r112890793