[FIXED] MQTT: more generic names for outgoing stream, etc. (#4484)

- [x] Branch rebased on top of current main (`git pull --rebase origin
dev`)
- [x] Changes squashed to a single commit (described
[here](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html))
 - [ ] Build is green in Travis CI

### Changes proposed in this pull request:

Removed `qos2` reference from the MQTT outgoing stream name and
subjects.

Rationale: As I was reviewing the documentation and thinking how to move
forward with MQTT, I am inclined to use the `$MQTT_out` stream and
session-specific consumers (as opposed to subscription-specific) for all
QOS-related packet delivery and PI persistence. I think I will be able
to repurpose it "as is", so proposing the generic name change.

Since these (stream, consumer, subscriptions) are newly introduced in
#4349 and have not yet been
released, this is a safe change.

server/mqtt.go

@@ -124,11 +124,12 @@ const (
 	mqttQoS2IncomingMsgsStreamName          = "$MQTT_qos2in"
 	mqttQoS2IncomingMsgsStreamSubjectPrefix = "$MQTT.qos2.in."
 
-	// Stream name and subjects for outgoing MQTT QoS2 PUBREL messages
-	mqttQoS2PubRelStreamName            = "$MQTT_qos2out"
-	mqttQoS2PubRelConsumerDurablePrefix = "$MQTT_PUBREL_"
-	mqttQoS2PubRelStoredSubjectPrefix   = "$MQTT.qos2.out."
-	mqttQoS2PubRelDeliverySubjectPrefix = "$MQTT.qos2.delivery."
+	// Stream name and subjects for outgoing MQTT QoS (PUBREL) messages
+	mqttOutStreamName               = "$MQTT_out"
+	mqttOutSubjectPrefix            = "$MQTT.out."
+	mqttPubRelSubjectPrefix         = "$MQTT.out.pubrel."
+	mqttPubRelDeliverySubjectPrefix = "$MQTT.deliver.pubrel."
+	mqttPubRelConsumerDurablePrefix = "$MQTT_PUBREL_"
 
 	// As per spec, MQTT server may not redeliver QoS 1 and 2 messages to
 	// clients, except after client reconnects. However, NATS Server will
@@ -1274,15 +1275,15 @@ func (s *Server) mqttCreateAccountSessionManager(acc *Account, quitCh chan struc
 		}
 	}
 
-	if si, err := lookupStream(mqttQoS2PubRelStreamName, "QoS2 outgoing PUBREL"); err != nil {
+	if si, err := lookupStream(mqttOutStreamName, "QoS2 outgoing PUBREL"); err != nil {
 		return nil, err
 	} else if si == nil {
 		// Create the stream for the incoming QoS2 messages that have not been
 		// PUBREL-ed by the sender. NATS messages are submitted as
 		// "$MQTT.pubrel.<session hash>"
 		cfg := &StreamConfig{
-			Name:      mqttQoS2PubRelStreamName,
-			Subjects:  []string{mqttQoS2PubRelStoredSubjectPrefix + ">"},
+			Name:      mqttOutStreamName,
+			Subjects:  []string{mqttOutSubjectPrefix + ">"},
 			Storage:   FileStorage,
 			Retention: InterestPolicy,
 			Replicas:  replicas,
@@ -2577,9 +2578,9 @@ func mqttSessionCreate(jsa *mqttJSA, id, idHash string, seq uint64, opts *Option
 		idHash:                 idHash,
 		seq:                    seq,
 		maxp:                   maxp,
-		pubRelSubject:          mqttQoS2PubRelStoredSubjectPrefix + idHash,
-		pubRelDeliverySubject:  mqttQoS2PubRelDeliverySubjectPrefix + idHash,
-		pubRelDeliverySubjectB: []byte(mqttQoS2PubRelDeliverySubjectPrefix + idHash),
+		pubRelSubject:          mqttPubRelSubjectPrefix + idHash,
+		pubRelDeliverySubject:  mqttPubRelDeliverySubjectPrefix + idHash,
+		pubRelDeliverySubjectB: []byte(mqttPubRelDeliverySubjectPrefix + idHash),
 	}
 }
 
@@ -2664,7 +2665,7 @@ func (sess *mqttSession) clear() error {
 		}
 	}
 	if pubRelDur != "" {
-		_, err := sess.jsa.deleteConsumer(mqttQoS2PubRelStreamName, pubRelDur)
+		_, err := sess.jsa.deleteConsumer(mqttOutStreamName, pubRelDur)
 		if isErrorOtherThan(err, JSConsumerNotFoundErr) {
 			return fmt.Errorf("unable to delete consumer %q for session %q: %v", pubRelDur, sess.id, err)
 		}
@@ -4463,10 +4464,10 @@ func (sess *mqttSession) ensurePubRelConsumerSubscription(c *client) error {
 		}
 
 		ccr := &CreateConsumerRequest{
-			Stream: mqttQoS2PubRelStreamName,
+			Stream: mqttOutStreamName,
 			Config: ConsumerConfig{
 				DeliverSubject: sess.pubRelDeliverySubject,
-				Durable:        mqttQoS2PubRelConsumerDurablePrefix + sess.idHash,
+				Durable:        mqttPubRelConsumerDurablePrefix + sess.idHash,
 				AckPolicy:      AckExplicit,
 				DeliverPolicy:  DeliverNew,
 				FilterSubject:  sess.pubRelSubject,