MQTT ping fixes

* Actually drop the connection when no response to ping arrives within the next interval.
 * Avoid sending immediate ping when wall time is adjusted, it's usually spurious.

PUBLISHED_FROM=8049280b58edfb94dd0fcb6a1e89ffefe69bcea1

src/mg_http.h

@@ -12,8 +12,8 @@
 
 #if MG_ENABLE_HTTP
 
-#include "mg_net.h"
 #include "common/mg_str.h"
+#include "mg_net.h"
 
 #ifdef __cplusplus
 extern "C" {
@@ -120,6 +120,9 @@ struct mg_ssi_call_ctx {
 #define MG_EV_HTTP_MULTIPART_REQUEST_END 125
 #endif
 
+#define MG_F_WEBSOCKET_NO_DEFRAG MG_F_PROTO_1
+#define MG_F_DELETE_CHUNK MG_F_PROTO_2
+
 /*
  * Attaches a built-in HTTP event handler to the given connection.
  * The user-defined event handler will receive following extra events: