Win. Use `uv_key_t` to store current thread handle in TLS on Windows prior Vista.

When we use LoadLibrary to load libuv library process gets AV when trying assign to `uv__current_thread`.

PS. To build it also needs comment `'_WIN32_WINNT=0x0600',` line in `uv.gyp`.
It doesn't really matter but it's common to place them somewhere near the relevant body of code.  That would be this file.  You could place it at the top, right after the includes.