Merge pull request #272 from wajam/clement-zk-replicas

Add ZookeeperConsistencyPersistence with replicas mapping
Fix build + typo
Merge pull request #273 from wajam/clement-fix-build

Fix build + typo
Fixes for easier ZookeeperConsistencyPersistence integration

* Fix a MatchError in service's Observer
* Fix type inference mess due to ???
* Use a pass-by-name explicitTokenMapping in ExplicitReplicaResolver