Event persistence for MQTT (#7268)

- The events will be persisted in queueStore if `queueDir` is set.
- Else, if queueDir is not set events persist in memory.

The events are replayed back when the mqtt broker is back online.

cmd/config-versions.go

@@ -884,7 +884,7 @@ type serverConfigV32 struct {
 	} `json:"policy"`
 }
 
-// serverConfigV33 is just like version '32', removes clientID from NATS and MQTT, and adds queueDir with MQTT.
+// serverConfigV33 is just like version '32', removes clientID from NATS and MQTT, and adds queueDir, queueLimit with MQTT.
 type serverConfigV33 struct {
 	quick.Config `json:"-"` // ignore interfaces