moved mac storage to singleton, tried to reduce response time

with these changes, the time until the client receives an ack from the host is ~100ms. i didn't have any issues storing/retrieving the mac address of the host in testing. Same for connecting a new client to the host

Apparently it's possible to increase the datarate used by espnow, however this could mess with reliability. i believe we should first try to decrease the amount of messages sent/reduce message size.

client/client_mock/src/main.cpp

@@ -16,14 +16,14 @@ void send_msgs(const std::__cxx11::list<Message> msgs) {
     for (const Message &msg: msgs) {
 
         if (msg.send() != ESP_OK) {
-            RtcMemory::store(msg.getMessageAsMinifiedJsonString());
+            RtcMemory::getInstance().store_data(msg.getMessageAsMinifiedJsonString());
         }
         unsigned long ts = millis();
         // it takes ~110ms for receiving an acknowledgement by the host in perfect conditions
         uint16_t message_timeout = 2000;
         while (!was_msg_received()) {
             if ((millis() - ts) > message_timeout) {
-                RtcMemory::store(msg.getMessageAsMinifiedJsonString());
+                RtcMemory::getInstance().store_data(msg.getMessageAsMinifiedJsonString());
                 ESP_LOGE(TAG, "Timeout: Host not available\n");
                 break;
             }