Core Consumer now has a RunningTask interface that wraps the underlying
Task interface so that the struct underlying the Task interface returned
by a Coordinator remains unchanged.

This allows users to implement coordinator-specific Task structs,
although Balancers and Handlers will have to type assert/switch the
metafora.Task to the cooordinator-specific implementation.

To allow for coordinator reuse with Task implementation variation, th
m_etcd.Coordinator allows users to define a custom NewTask function to
unmarshal the etcd task node into a use case specific struct.