allow injection of external libuv event loop

[not-implemented]

For integrating redis-plus-plus directly into existing libuv event loops, the loop should be injectable.

I tried it now via EventLoop::EventLoop(uv_loop_t* external_loop) which seems to work fine also in production.

The second commit implements EventLoop.drain() for a graceful shutdown which is needed to cleanly exit the external loop.

Is there any chance to get it merged to upstream? :-)