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
Get rid of `replicas=` list in Zookeeper

Use children of the `replicas` node instead.
Fix indentation and pattern match naming
Remove useless toMap
Merge pull request #289 from wajam/clement-rm-replicas-node

Get rid of `replicas=` list in Zookeeper