store-worker: implement

store-worker is a thread + async queue to throttle requests to a single thread.

lib/mu-store.cc

@@ -131,6 +131,7 @@ struct Store::Private {
 	XapianDb xapian_db_;
 	Config config_;
 	ContactsCache            contacts_cache_;
+	std::unique_ptr<StoreWorker> store_worker_;
 	std::unique_ptr<Indexer> indexer_;
 
 	const std::string	root_maildir_;
@@ -252,6 +253,7 @@ Store::Store(Store&& other)
 {
 	priv_ = std::move(other.priv_);
 	priv_->indexer_.reset();
+	priv_->store_worker_.reset();
 }
 
 Store::~Store() = default;
@@ -316,6 +318,15 @@ Store::indexer()
 	return *priv_->indexer_.get();
 }
 
+StoreWorker&
+Store::store_worker()
+{
+	if (!priv_->store_worker_)
+		priv_->store_worker_ = std::make_unique<StoreWorker>(*this);
+
+	return *priv_->store_worker_;
+}
+
 Result<Store::Id>
 Store::add_message(Message& msg, bool is_new)
 {