Update mqtt proxy to broker (#876)

* make mqttproxy a broker

* add more test and optim code

* udpate mqtt test

* optim code for mqttproxy

* update broker to eliminite possibility of concurrently run connectWatcher

* mqttproxy: optim store process, solve client connect to multi easegress instance conflict, optim resend time interval, and other updates based on comments

* refactor send msg part in broker mode

* update test

* update test

* optim mqtt client process publish

* optim code for processBrokerModePublish

* update doc of mqtt proxy

* update broker.go

doc/cookbook/mqtt-proxy.md

@@ -23,6 +23,7 @@
 - As `Pipeline` is protocol independent, it can use MQTT filters to do things like user authentication or topic mapping (map MQTT multi-level topic into single topic and key-value headers).
 - We also support MQTT clients to `subscribe` topics (wildcard is supported) and send messages back to the MQTT clients through the HTTP endpoint.
 
+By default:
 ```
  publish msg publish pipeline
 MQTT client ------------> Easegress MQTTProxy ----------------> Backend like Kafka
@@ -36,6 +37,18 @@ all msg send back to MQTT clients come from HTTP endpoint.
 ```
 - We assume that IoT devices (use MQTT client) report their status to the backend (through tools like Kafka), and backend process these messages and send instructions back to IoT devices.
 
+Using `brokerMode`:
+```
+ publish msg publish pipeline
+MQTT client ------------> Easegress MQTTProxy ----------------> Backend like Kafka
+ |
+ | also send msg to subscribers
+ |
+ subscribe msg send msg through http endpoint
+MQTT client <------------ Easegress <------------------------------- Backend Server
+```
+By setting `brokerMode` to `true`. MQTTProxy can both send msg to backend and subscribers. Users can also send msg to clients by using HTTP endpoint.
+
 # Example
 Save following yaml to file `mqttproxy.yaml` and then run
 ```bash
@@ -60,6 +73,8 @@ rules:
 - when:
  packetType: Publish
  pipeline: pipeline-mqtt-publish
+# by default, brokerMode is disabled. 
+brokerMode: true
 
 ---