Get rid of `replicas=` list in Zookeeper

`ZookeeperConsistencyPersistence` will now be looking at the children of the `replicas` node instead of the value of this node, as the information is contained in the hierarchy itself. Replicas will have to be set this way before starting the cluster.

Before:
```/services/service.app.org/members/4294967294/replicas=host:nrv=9898|host:nrv=9899```

After:
```/services/service.app.org/members/4294967294/replicas/host:9898
/services/service.app.org/members/4294967294/replicas/host:9899```

Other changes:
- refactor replicas mapping persistence using an Agent
- rename all agents update methods using `update` prefix
- add updateMemberReplicasThenLags method to update only values that
may have changed regarding a specific service member
- check for existence of replicas path before trying a get() to avoid a
potential race