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