Improve memory of producer retry queue

The old version just used a simple slice as a queue, using append and slice[1:].
This had correct (amortized O(1)) running time, but did not behave well with the
garbage collector. In particular:
- elements removed from the slice were still pointed to by the underlying array,
  preventing them from being collected until the slice was resized
- the slice would only be resized when its current capacity was "full", even if
  it had a substantial amount of free space it was wasting

Switch instead to my queue implementation (which I wrote a while ago to solve
these exact problems somewhere else). It uses a ringbuffer that is guaranteed to
stay with a factor of 2 size of the actual number of messages stored, it
generates a lot less garbage when the input and output rates are about the same,
and it nils removed elements in the underlying array so the garbage collector
can pick them up at its leisure.

The only possible concern is that it uses `interface{}` but the usage is so
restrained that type-safety is trivial and the performance will not be noticable
given everything else the producer has to do.

async_producer.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/eapache/go-resiliency/breaker"
+	"github.com/eapache/queue"
 )
 
 func forceFlushThreshold() int {
@@ -592,19 +593,21 @@ func (p *asyncProducer) flusher(broker *Broker, input chan []*ProducerMessage) {
 // effectively a "bridge" between the flushers and the topicDispatcher in order to avoid deadlock
 // based on https://godoc.org/github.com/eapache/channels#InfiniteChannel
 func (p *asyncProducer) retryHandler() {
-	var buf []*ProducerMessage
-	var msg *ProducerMessage
-	refs := 0
-	shuttingDown := false
+	var (
+		msg          *ProducerMessage
+		buf          = queue.New()
+		refs         = 0
+		shuttingDown = false
+	)
 
 	for {
-		if len(buf) == 0 {
+		if buf.Length() == 0 {
 			msg = <-p.retries
 		} else {
 			select {
 			case msg = <-p.retries:
-			case p.input <- buf[0]:
-				buf = buf[1:]
+			case p.input <- buf.Peek().(*ProducerMessage):
+				buf.Remove()
 				continue
 			}
 		}
@@ -622,13 +625,14 @@ func (p *asyncProducer) retryHandler() {
 				break
 			}
 		} else {
-			buf = append(buf, msg)
+			buf.Add(msg)
 		}
 	}
 
 	close(p.retries)
-	for i := range buf {
-		p.input <- buf[i]
+	for buf.Length() != 0 {
+		p.input <- buf.Peek().(*ProducerMessage)
+		buf.Remove()
 	}
 	close(p.input)
 }