Add server_keepalive config

etc/emqx.conf

@@ -580,6 +580,11 @@ zone.external.enable_stats = on
 ## Value: boolean
 ## zone.external.shared_subscription = false
 
+## Server Keep Alive
+##
+## Value: Number
+## zone.external.server_keepalive = 0
+
 ## The backoff for MQTT keepalive timeout. The broker will kick a connection out
 ## until 'Keepalive * backoff * 2' timeout.
 ##

priv/emqx.schema

@@ -735,6 +735,11 @@ end}.
   {datatype, {enum, [true, false]}}
 ]}.
 
+%% @doc Server Keepalive
+{mapping, "zone.$name.server_keepalive", "emqx.zones", [
+  {datatype, integer}
+]}.
+
 %% @doc Keepalive backoff
 {mapping, "zone.$name.keepalive_backoff", "emqx.zones", [
   {default, 0.75},