[core] Introduce Scheduler::GetSequenced() API

The newly introduced `Scheduler::GetSequenced()` returns
sequenced schedulers from the cache limited to 10 instances,
preventing from spawning too many threads.

test/util/async_task.test.cpp

@@ -169,4 +169,47 @@ TEST(AsyncTask, scheduleAndReplyValue) {
     auto sheduler = Scheduler::GetBackground();
     sheduler->scheduleAndReplyValue(runInBackground, onResult);
     loop.run();
+}
+
+TEST(AsyncTask, SequencedScheduler) {
+    RunLoop loop;
+    std::thread::id caller_id = std::this_thread::get_id();
+    std::thread::id bg_id;
+    int count = 0;
+
+    auto first = [caller_id, &bg_id, &count]() {
+        EXPECT_EQ(0, count);
+        bg_id = std::this_thread::get_id();
+        EXPECT_NE(caller_id, bg_id);
+        count++;
+    };
+    auto second = [&bg_id, &count]() {
+        EXPECT_EQ(1, count);
+        EXPECT_EQ(bg_id, std::this_thread::get_id());
+        count++;
+    };
+    auto third = [&bg_id, &count, &loop]() {
+        EXPECT_EQ(2, count);
+        EXPECT_EQ(bg_id, std::this_thread::get_id());
+        loop.stop();
+    };
+
+    auto sheduler = Scheduler::GetSequenced();
+
+    sheduler->schedule(first);
+    sheduler->schedule(second);
+    sheduler->schedule(third);
+    loop.run();
+}
+
+TEST(AsyncTask, MultipleSequencedSchedulers) {
+    std::vector<std::shared_ptr<Scheduler>> shedulers;
+
+    for (int i = 0; i < 10; ++i) {
+        auto scheduler = Scheduler::GetSequenced();
+        EXPECT_TRUE(std::none_of(
+            shedulers.begin(), shedulers.end(), [&scheduler](const auto &item) { return item == scheduler; }));
+        shedulers.emplace_back(std::move(scheduler));
+    }
+    EXPECT_EQ(shedulers.front(), Scheduler::GetSequenced());
 }