add a streaming CID set

used in https://github.com/ipfs/go-ipfs/pull/4804

set.go

 package cid
 
+import (
+	"context"
+)
+
 // Set is a implementation of a set of Cids, that is, a structure
 // to which holds a single copy of every Cids that is added to it.
 type Set struct {
@@ -65,3 +69,34 @@ func (s *Set) ForEach(f func(c *Cid) error) error {
 	}
 	return nil
 }
+
+// StreamingSet is an extension of Set which allows to implement back-pressure
+// for the Visit function
+type StreamingSet struct {
+	Set *Set
+	New chan *Cid
+}
+
+// NewStreamingSet initializes and returns new Set.
+func NewStreamingSet() *StreamingSet {
+	return &StreamingSet{
+		Set: NewSet(),
+		New: make(chan *Cid),
+	}
+}
+
+// Visitor creates new visitor which adds a Cids to the set and emits them to
+// the set.New channel
+func (s *StreamingSet) Visitor(ctx context.Context) func(c *Cid) bool {
+	return func(c *Cid) bool {
+		if s.Set.Visit(c) {
+			select {
+			case s.New <- c:
+			case <-ctx.Done():
+			}
+			return true
+		}
+
+		return false
+	}
+}