to*Map with merge function and grouping* with ordered downstream: allow concurrent collection for unordered source

[amaembo]

Methods StreamEx.toMap(keyMapper, valMapper, mergeFunction), StreamEx.toSortedMap(keyMapper, valMapper, mergeFunction), EntryStream.toMap(mergeFunction), EntryStream.toSortedMap(mergeFunction), EntryStream.toCustomMap(mergeFunction, mapSupplier) do not use concurrent collection for parallel stream as mergeFunction is not guaranteed to be commutative. However if stream is unordered it's still possible to use concurrent collection, so it should be implemented. This way user may explicitly call .unordered() to speed up the results.

The same applies for EntryStream.grouping(downstream), EntryStream.grouping(supplier, downstream), StreamEx.groupingBy(classifier, downstream), StreamEx.groupingBy(classifier, supplier, downstream): currently parallel concurrent collection is used only for unordered downstream collector. It could be used also for unordered source.