uv: fix performance issue

unix implementation of uv_async_t has been wasting cpu cycles for nothing since 1.29.0 release
implement efficient callback scheduling for linux

src/base/io/Async.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <uv.h>
+
+
+// since 2019.05.16, Version 1.29.0 (Stable)
+#if (UV_VERSION_MAJOR >= 1) && (UV_VERSION_MINOR >= 29) && defined(__linux__)
+#define XMRIG_UV_PERFORMANCE_BUG
+namespace xmrig {
+
+
+struct uv_async_t: uv_poll_t
+{
+    typedef void (*uv_async_cb)(uv_async_t* handle);
+    ~uv_async_t();
+    int m_fd = -1;
+    uv_async_cb m_cb = nullptr;
+};
+
+
+using uv_async_cb = uv_async_t::uv_async_cb;
+extern int uv_async_init(uv_loop_t *loop, uv_async_t *async, uv_async_cb cb);
+extern int uv_async_send(uv_async_t *async);
+
+
+} // namespace xmrig
+#endif