Add 'mqtt.conn.force_gc_count' config to tune GC of MQTT connection

etc/emq.conf

@@ -106,6 +106,13 @@ mqtt.max_clientid_len = 1024
 ## Max Packet Size Allowed, 64K by default.
 mqtt.max_packet_size = 64KB
 
+##--------------------------------------------------------------------
+## MQTT Connection
+##--------------------------------------------------------------------
+
+## Force GC: pos_integer
+mqtt.conn.force_gc_count = 100
+
 ##--------------------------------------------------------------------
 ## MQTT Client
 ##--------------------------------------------------------------------

priv/emq.schema

@@ -316,6 +316,15 @@ end}.
    {max_packet_size,  cuttlefish:conf_get("mqtt.max_packet_size", Conf)}]
 end}.
 
+%%--------------------------------------------------------------------
+%% MQTT Connection
+%%--------------------------------------------------------------------
+
+%% @doc Force the client to GC: integer
+{mapping, "mqtt.conn.force_gc_count", "emqttd.conn_force_gc_count", [
+  {datatype, integer}
+]}.
+
 %%--------------------------------------------------------------------
 %% MQTT Client
 %%--------------------------------------------------------------------