rafthtt: smart batching

Improved the overall performance more than 20% under heavyload
with little latency impact

heavy load
```
Requests/sec: ~23200  vs  Requests/sec: ~31500

Latency distribution:
  10% in 0.0883 secs.
  25% in 0.1022 secs.
  50% in 0.1207 secs.
  75% in 0.1460 secs.
  90% in 0.1647 secs.
  95% in 0.1783 secs.
  99% in 0.2223 secs.

vs

Latency distribution:
  10% in 0.1119 secs.
  25% in 0.1272 secs.
  50% in 0.1469 secs.
  75% in 0.1626 secs.
  90% in 0.1765 secs.
  95% in 0.1863 secs.
  99% in 0.2276 secs.
```

Similar on light load too.

rafthttp/stream.go

@@ -136,6 +136,7 @@ func (cw *streamWriter) run() {
 		t          streamType
 		enc        encoder
 		flusher    http.Flusher
+		batched    int
 	)
 	tickc := time.Tick(ConnReadTimeout / 3)
 
@@ -146,6 +147,7 @@ func (cw *streamWriter) run() {
 			err := enc.encode(linkHeartbeatMessage)
 			if err == nil {
 				flusher.Flush()
+				batched = 0
 				reportSentDuration(string(t), linkHeartbeatMessage, time.Since(start))
 				continue
 			}
@@ -159,7 +161,13 @@ func (cw *streamWriter) run() {
 			start := time.Now()
 			err := enc.encode(m)
 			if err == nil {
-				flusher.Flush()
+				if len(msgc) == 0 || batched > streamBufSize/2 {
+					flusher.Flush()
+					batched = 0
+				} else {
+					batched++
+				}
+
 				reportSentDuration(string(t), m, time.Since(start))
 				continue
 			}