Improved regular scroll api by using IndexSearch#searchAfter instead of regular search methods.

This prevents the creation of priority queues of `from + size`, instead the size of the priority queue will always be equal to `size`.

Closes #4940
Implemented compareDocToValue() in NestedFieldComparator
Implemented @jpountz feedback points
Added classicScroll that does scroll as was done before version 1.1.0 and is enabled when one of the nodes the request being send to is on version lower then 1.1.0 Added duel tests.
Properly implement BytesRefOrdValComparator#compareTop(int) and BytesRefOrdValComparator#compareTopMissing()
Cut over to use `tests.mock.version` to start nodes on older version to test the classic scroll behaviour (which uses `from` instead of searchAfter)