Graceful stop is not possible with enableKeepAlive
Hi.

As much as I have seen throughout examples and issue comments, the only advertised way to stop running server is to call `EventLoop::stop()`. I agree that it's a perfectly viable solution, however ReactPHP docs [state](https://github.com/reactphp/event-loop/tree/85a0b7c0e35a47387a61d2ba8a772a7855b6af86#stop) that usage of this method is not recommended to stop the loop, and there are projects that rely on automatic loop termination istead.

The problem I encountered is that `enableKeepAlive()` created periodic timer that is never ever cancelled, resulting in websocket server never terminating without call to `EventLoop::stop()` (which I do not rely on).

I would suggest adding another method that would disable keep-alive and cancel periodic timer.

A workaround I use is to create a timer with 0.1 second timeout that would call `EventLoop::stop()`. But this is not as alegant as it could be.