Add support for callbacks on exists() in ZookeeperClient
Add watches for create/update of lag nodes in ZookeeperConsistencyPersistence
Redirect traffic to slaves when master is down, depending on replication lag
Syntax changes + use eventually instead of sleeps in tests
Refactor replication lag persistence using an Agent
Rename watchers and callbacks caches
Avoid potential race while fetching a replication lag value
Merge pull request #284 from wajam/clement-tx-delta

Redirect traffic to slaves when master is down
Merge branch 'master' into clement-change-master

Conflicts:
	nrv-zookeeper/src/main/scala/com/wajam/nrv/zookeeper/consistency/ZookeeperConsistencyPersistence.scala