speed up net write if socket buffer large enough

add uv_try_write() to libuv, try to write as many bytes as possible
into socket buffer for nonblocking socket, if all data is written
successfully, async uv write can be avoid, thereby saving the
consumption of uv context and queue operations.

I've opened an issue in node, where explained the reason in detail:
https://github.com/joyent/node/issues/4699

And the corresponding patch for node:
https://github.com/freedaxin/node/commit/d0718d00a231a69c78862bfadab0c7b7b5c59122