Add thread pool async I/O backend (#97)

* Add thread pool async I/O backend

* Make it work with older code

src/async_io.h

@@ -24,6 +24,7 @@ namespace detail {
 #if WIBO_ENABLE_LIBURING
 std::unique_ptr<AsyncIOBackend> createIoUringBackend();
 #endif
+std::unique_ptr<AsyncIOBackend> createThreadPoolBackend();
 
 } // namespace detail