Added TaskHelpers.FromResult(bool)

- no need to do cond ? TrueTask : FalseTask anymore
Moved default LINQ operators implementation on AsyncIterator<T> itself, so no need to impl it everywhere
Refactored MergeSort iterator into a base "Merge" iterator, added an Intersect merge iterator

- FdbQueryMergeIterator handles iterator over multiple iterators at at time
- FdbMergeSortSelectable only handle the MergeSort part (output items in order)
- FdbIntersectIterator only output items when it is contained in all the sub iterators
- Move range unit test into separate RangeQueryFacts