feat: Make state and size atomic

This makes the mqtt client state atomic to reduce the scope of locking in
some parts of the code.

lib/include/mqtt_client_priv.h

@@ -114,7 +114,7 @@ struct esp_mqtt_client {
     esp_transport_handle_t transport;
     mqtt_config_storage_t *config;
     mqtt_state_t  mqtt_state;
-    mqtt_client_state_t state;
+    _Atomic mqtt_client_state_t state;
     uint64_t refresh_connection_tick;
     int64_t keepalive_tick;
     uint64_t reconnect_tick;

lib/mqtt_outbox.c

@@ -24,7 +24,7 @@ typedef struct outbox_item {
 STAILQ_HEAD(outbox_list_t, outbox_item);
 
 struct outbox_t {
-    uint64_t size;
+    _Atomic uint64_t size;
     struct outbox_list_t *list;
 };