This adds coordination between DistributedHerders using the generalized consumer
support, allowing automatic balancing of connectors and tasks across workers. A
few pieces that require interaction between workers (resolving config
inconsistencies, forwarding of configuration changes to the leader worker) are
incomplete because they require REST API support to implement properly.
Author: Ewen Cheslack-Postava <me@ewencp.org>
Reviewers: Jason Gustafson, Gwen Shapira
Closes#321 from ewencp/kafka-2371-distributed-herder