docs: public void cleanExpiredCallbacks() {
          long now = System.currentTimeMillis();
          
          // Cuncurrency could be possible for multiple instantiations
          synchronized(regCallbacks) {    
              Iterator<Map.Entry<String, RegisteredCallback>> it = regCallbacks.entrySet().iterator();
              Map.Entry<String, RegisteredCallback> entry;
              
              while (it.hasNext()) {
                  entry = it.next();
                  if (now - entry.getValue().getTimestamp() > CALLBACK_EXPIRE_TIME) {
                      it.remove();
                  }
              }
          }
      }