func (s *Segment) SetNodeCallback(fn NodeCallback) {
	s.callb = fn
}
SetNodeCallback sets callback for segment builder
package skiplist
import "math/rand"
import "unsafe"
type NodeCallback func(*Node)
type Segment struct {
	builder *Builder
	tail    []*Node
	head    []*Node
	rand    *rand.Rand
	callb   NodeCallback
	count   uint64
	sts Stats
}