feature(mobile): sync assets, albums & users to local database on device (#1759)

* feature(mobile): sync assets, albums & users to local database on device

* try to fix tests

* move DB sync operations to new SyncService

* clear db on user logout

* fix reason for endless loading timeline

* fix error when deleting album

* fix thumbnail of device albums

* add a few comments

* fix Hive box not open in album service when loading local assets

* adjust tests to int IDs

* fix bug: show all albums when Recent is selected

* update generated api

* reworked Recents album isAll handling

* guard against wrongly interleaved sync operations

* fix: timeline asset ordering (sort asset state by created at)

* fix: sort assets in albums by created at

mobile/lib/utils/async_mutex.dart

@@ -0,0 +1,16 @@
+import 'dart:async';
+
+/// Async mutex to guarantee actions are performed sequentially and do not interleave
+class AsyncMutex {
+  Future _running = Future.value(null);
+
+  /// Execute [operation] exclusively, after any currently running operations.
+  /// Returns a [Future] with the result of the [operation].
+  Future<T> run<T>(Future<T> Function() operation) {
+    final completer = Completer<T>();
+    _running.whenComplete(() {
+      completer.complete(Future<T>.sync(operation));
+    });
+    return _running = completer.future;
+  }
+}