- Redefine isConnected in terms of nimbleBluetoothConnHandle. isConnected was not safe because BLEServer::getConnectedCount is not thread-safe (https://github.com/espressif/arduino-esp32/issues/12538) while isConnected is called from various threads. Now we can avoid checking bleServer every time before calling isConnected. - getRssi: don't try to "populate nimbleBluetoothConnHandle", that requires calling BLEServer::getPeerDevices which is not thread-safe (same issue as above).