add GeventRabbitmqBroker

[horpto]

Under gevent dramatiq is opening a connection at each query (with message).

[horpto]

@Bogdanp if I'd remove GeventRabbitmqBroker itself and keep other changes as refactoring can it be merged?

[Bogdanp]

Thanks, but I don't think the additional hooks are needed since someone can extend the class and override the channel and connection properties instead.