lep: pull based event loop

This is what I kind-of revolved around in https://github.com/libuv/libuv/issues/6#issuecomment-64758337 but that's not exactly what they want.

Anyway, I don't disagree that it could be useful. Now, do we have an actual use case which we need to cover? The two approaches aren't orthogonal, so this could be refined in a follow-up LEP if we see the need. Or we could amend this one as something to consider in the future and then just edit the LEP and that's that.

We could have:

~~~~
uv_req_t* uv_backend_dequeue(uv_loop_t* loop);
void uv_backend_dispatch(uv_req_t* req);
void uv_backend_dispatch_all(uv_loop_t* loop);
~~~~