Fix CPU load of dockerapi container

Previously the handle_pubsub_messages() loop was executing every 10ms
when there was no message available. Now reading from the redis network
socket will block (the coroutine) for up to 30s before it returns when
no message is available.

Using channel.listen() would be even better, but it lacks the
ignore_subscribe_messages option and I could not figure out how to
filter the returned messages.