Adding "min" score mode to parent-child queries

Support for "max", "sum", and "avg" already existed.

Closes #7603 
I don't see NO_MORE_DOCS changing in the future.

I don't dislike having NO_MORE_DOCS=MAX_VALUE, it makes the sequences of integers returned by DocIdSetIterator monotonic from -1 (not started) to MAX_VALUE (exhausted) :)