Eliminate requirement to use ZK for leadership

[cer]

Problem

• The Eventuate CDC must be configured with a ZK connection string.
• This is a problem in an environment that uses secure communication, see #53

Solution

• Use Kafka topic as the coordination mechanism

[cer]

@klausvahter did your team ever resolve this issue. It's something I need to implement ASAP.

[rrrship]

Our ZK is set up internally without any additional auth enabled on ZK side, so we haven't had any issues with that. Kafka uses certs separately, but this is working fine.

But we're interested in this ticket's results as well, only because of the overhead of managing ZK only for our outboxes.