feat: Re-introduce locking the ratchet map/device list

This makes the locking much more intelligent, allowing us
to encrypt to/decrypt from groups while still being able to
bypass the lock for unaffiliated JIDs.

test/queue_test.dart

@@ -0,0 +1,56 @@
+import 'dart:async';
+
+import 'package:omemo_dart/src/omemo/queue.dart';
+import 'package:test/test.dart';
+
+Future<void> testMethod(RatchetAccessQueue queue, List<String> data, int duration) async {
+  await queue.enterCriticalSection(data);
+
+  await Future<void>.delayed(Duration(seconds: duration));
+
+  await queue.leaveCriticalSection(data);
+}
+
+void main() {
+  test('Test blocking due to conflicts', () async {
+    final queue = RatchetAccessQueue();
+
+    unawaited(testMethod(queue, ['a', 'b', 'c'], 5));
+    unawaited(testMethod(queue, ['a'], 4));
+
+    await Future<void>.delayed(const Duration(seconds: 1));
+    expect(
+      queue.runningOperations.containsAll(['a', 'b', 'c']),
+      isTrue,
+    );
+    expect(queue.runningOperations.length, 3);
+
+    await Future<void>.delayed(const Duration(seconds: 4));
+
+    expect(
+      queue.runningOperations.containsAll(['a']),
+      isTrue,
+    );
+    expect(queue.runningOperations.length, 1);
+
+    await Future<void>.delayed(const Duration(seconds: 4));
+    expect(queue.runningOperations.length, 0);
+  });
+
+  test('Test not blocking due to no conflicts', () async {
+    final queue = RatchetAccessQueue();
+
+    unawaited(testMethod(queue, ['a', 'b'], 5));
+    unawaited(testMethod(queue, ['c'], 5));
+    unawaited(testMethod(queue, ['d'], 5));
+
+    await Future<void>.delayed(const Duration(seconds: 1));
+    expect(queue.runningOperations.length, 4);
+    expect(
+      queue.runningOperations.containsAll([
+        'a', 'b', 'c', 'd',
+      ]),
+      isTrue,
+    );
+  });
+}