mqtt: fix relay dependency and some comments

Plus, heartbeat will send this regardless

For the clean session stuff... when changing configuration of something like RPN,
the old subscription never goes away and we also receive old messages when QoS > 0 is used

Pending changes in the MQTT client(s) to make this work more reliably

code/espurna/config/general.h

@@ -444,10 +444,6 @@
 #define RELAY_SAVE_DELAY            1000
 #endif
 
-#ifndef RELAY_REPORT_STATUS
-#define RELAY_REPORT_STATUS         1
-#endif
-
 // Configure the MQTT payload for ON, OFF and TOGGLE
 #ifndef RELAY_MQTT_OFF
 #define RELAY_MQTT_OFF              "0"

code/espurna/mqtt.cpp

@@ -196,6 +196,20 @@ SecureClientConfig _mqtt_sc_config {
 // Client configuration & setup
 // -----------------------------------------------------------------------------
 
+// TODO: MQTT standard has some weird rules about session persistance on the broker
+// ref. 3.1.2.4 Clean Session, where we are uniquely identified by the client-id:
+// - subscriptions that are no longer useful are still there
+//   unsub # will be acked, but we were never subbed to # to begin with ...
+// - we *will* receive messages that were sent using qos 1 or 2 while we were offline
+//   which is only sort-of good, but MQTT broker v3 will never timeout those messages.
+//   this would be the main reason for turning ON the clean session
+// - connecting with clean session ON will purge existing session *and* also prevent
+//   the broker from caching the messages after the current connection ends.
+//   there is no middle-ground, where previous session is removed but the current one is preserved
+//   so, turning it ON <-> OFF during runtime is not very useful :/
+//
+// Pending MQTT v5 client
+
 #if MQTT_LIBRARY == MQTT_LIBRARY_ASYNCMQTTCLIENT
 
 void _mqttSetupAsyncClient(bool secure = false) {
@@ -204,6 +218,7 @@ void _mqttSetupAsyncClient(bool secure = false) {
     _mqtt.setClientId(_mqtt_clientid.c_str());
     _mqtt.setKeepAlive(_mqtt_keepalive);
     _mqtt.setCleanSession(false);
+
     _mqtt.setWill(_mqtt_will.c_str(), _mqtt_qos, _mqtt_retain, _mqtt_payload_offline.c_str());
 
     if (_mqtt_user.length() && _mqtt_pass.length()) {
@@ -349,7 +364,7 @@ void _mqttConfigure() {
     {
         String setter = getSetting("mqttSetter", MQTT_SETTER);
         String getter = getSetting("mqttGetter", MQTT_GETTER);
-        bool forward = !setter.equals(getter) && RELAY_REPORT_STATUS;
+        bool forward = !setter.equals(getter);
 
         _mqttApplySetting(_mqtt_setter, setter);
         _mqttApplySetting(_mqtt_getter, getter);