rafthttp: use buffered channel as recv/prop chan

So it ensures that the message will not be discarded because the receive
side has not been ready, which happens easily in multiple core test.

Use log.fatal instead of log.error. The test exits when there is
something wrong because the error may affect following test cases.

rafthttp/stream_test.go

@@ -221,8 +221,8 @@ func TestStreamReaderDialDetectUnsupport(t *testing.T) {
 // TestStream tests that streamReader and streamWriter can build stream to
 // send messages between each other.
 func TestStream(t *testing.T) {
-	recvc := make(chan raftpb.Message)
-	propc := make(chan raftpb.Message)
+	recvc := make(chan raftpb.Message, streamBufSize)
+	propc := make(chan raftpb.Message, streamBufSize)
 	msgapp := raftpb.Message{
 		Type:    raftpb.MsgApp,
 		From:    2,
@@ -294,10 +294,10 @@ func TestStream(t *testing.T) {
 		select {
 		case m = <-tt.wc:
 		case <-time.After(time.Second):
-			t.Errorf("#%d: failed to receive message from the channel", i)
+			t.Fatalf("#%d: failed to receive message from the channel", i)
 		}
 		if !reflect.DeepEqual(m, tt.m) {
-			t.Errorf("#%d: message = %+v, want %+v", i, m, tt.m)
+			t.Fatalf("#%d: message = %+v, want %+v", i, m, tt.m)
 		}
 	}
 }