Defer web requests if JSON lock contended

Use the web server's queuing mechanism to call us back later.

wled00/json.cpp

@@ -1060,7 +1060,7 @@ void serveJson(AsyncWebServerRequest* request)
   }
 
   if (!requestJSONBufferLock(17)) {
-    serveJsonError(request, 503, ERR_NOBUF);
+    request->deferResponse();    
     return;
   }
   // releaseJSONBufferLock() will be called when "response" is destroyed (from AsyncWebServer)

wled00/set.cpp

@@ -628,7 +628,10 @@ void handleSettingsSet(AsyncWebServerRequest *request, byte subPage)
   //USERMODS
   if (subPage == SUBPAGE_UM)
   {
-    if (!requestJSONBufferLock(5)) return;
+    if (!requestJSONBufferLock(5)) {
+      request->deferResponse();
+      return;
+    }
 
     // global I2C & SPI pins
     int8_t hw_sda_pin  = !request->arg(F("SDA")).length() ? -1 : (int)request->arg(F("SDA")).toInt();

wled00/wled_server.cpp

@@ -288,7 +288,7 @@ void initServer()
     bool isConfig = false;
 
     if (!requestJSONBufferLock(14)) {
-      serveJsonError(request, 503, ERR_NOBUF);
+      request->deferResponse();
       return;
     }