common/detached_tasks: Wait for tasks before shutting down

If this is not waited on, the synchronization primitives are destroyed
whe main exits and the detached task ends up signalling garbage and not
properly finishing.

src/common/detached_tasks.cpp

@@ -21,6 +21,8 @@ void DetachedTasks::WaitForAllTasks() {
 }
 
 DetachedTasks::~DetachedTasks() {
+    WaitForAllTasks();
+
     std::unique_lock lock{mutex};
     ASSERT(count == 0);
     instance = nullptr;