Update remote characteristic value from a notification

* Add NimBLERemoteCharacteristic::getValue(time_t *timestamp = nullptr) to get the latest remote characteristic and (optionally) it's timestamp.

* Added a timestamp to NimBLEAdvertisedDevice for the moment a device was scanned

src/NimBLEDevice.cpp

@@ -40,7 +40,7 @@ static const char* LOG_TAG = "NimBLEDevice";
 /**
  * Singletons for the NimBLEDevice.
  */
-bool            initialized = false;
+static bool            initialized = false;
 #if defined(CONFIG_BT_NIMBLE_ROLE_OBSERVER)
 NimBLEScan*     NimBLEDevice::m_pScan = nullptr;
 #endif