Connection established, Kafka unavailable

Hi,

We're facing an issue where we send and receive messages using aiokafka. When restarting kafka docker container the connection is established, but kafka seems not to be ready yet, returning empty metadata, causing sytem to hang. After further debugging I figured that adding a sleep of few ms before [this line](https://github.com/aio-libs/aiokafka/blob/master/aiokafka/client.py#L294) solves the problem. I am not sure this is the right solution. If so, I can open a pull request with the fix. Otherwise, I'd appreciate if you could come up with a more appropriate one @progovoy

Thanks,