func (s *Skiplist) findSpliceForLevel(key []byte, before *node, level int) (*node, *node) {
	for {
		// Assume before.key < key.
		next := s.getNext(before, level)
		if next == nil {
			return before, next
		}
		nextKey := next.key(s.arena)
		cmp := y.CompareKeys(key, nextKey)
		if cmp == 0 {
			// Equality case.
			return next, next
		}
		if cmp < 0 {
			// before.key < key < next.key. We are done for this level.
			return before, next
		}
		before = next // Keep moving right on this level.
	}
}
func (s *Skiplist) findSpliceForLevel(key []byte, before *node, level int) (*node, *node) {
	for {
		// Assume before.key < key.
		next := s.getNext(before, level)
		if next == nil {
			return before, next
		}
		nextKey := next.key(s.arena)
		cmp := y.CompareKeys(key, nextKey)
		if cmp == 0 {
			// Equality case.
			return next, next
		}
		if cmp < 0 {
			// before.key < key < next.key. We are done for this level.
			return before, next
		}
		before = next // Keep moving right on this level.
	}
}
// findSpliceForLevel returns (outBefore, outAfter) with outBefore.key <= key <= outAfter.key.
// The input "before" tells us where to start looking.
// If we found a node with the same key, then we return outBefore = outAfter.
// Otherwise, outBefore.key < key < outAfter.key.