use asynchronous worker termination

instead of waiting until all threads have joined, we are now supplying a callback that will be invoked once all threads have terminated. you can use this callback to free the worker struct

src/storage/sqlite_store.cpp

@@ -39,8 +39,9 @@ SQLiteStore::~SQLiteStore() {
     // Nothing to do. This function needs to be here because we're forward-declaring
     // Database, so we need the actual definition here to be able to properly destruct it.
     if (worker) {
-        uv_worker_close(worker);
-        delete worker;
+        uv_worker_close(worker, [](uv_worker_t *worker) {
+            delete worker;
+        });
     }
 }