Change the default value of mqtt.keepalive_backoff to 0.75

etc/emq.conf

@@ -1,5 +1,5 @@
 ##====================================================================
-## EMQ Configuration R2.3.0
+## EMQ Configuration R2.3.3
 ##====================================================================
 
 ##--------------------------------------------------------------------
@@ -196,7 +196,7 @@ mqtt.max_packet_size = 64KB
 mqtt.websocket_protocol_header = on
 
 ## The Keepalive timeout: Keepalive * backoff * 2
-mqtt.keepalive_backoff = 1.25
+mqtt.keepalive_backoff = 0.75
 
 ##--------------------------------------------------------------------
 ## MQTT Connection