For TEST
Add ZooKeeperActor.

ZooKeeperActor is dedicated worker to run ZooKeeper client API.
This actor is helpero use ZooKeeper API asynchronously and safe.

ZooKeeperActor provides two features.
 1. Pending commands if the connection is not available.

  ZooKeeperActor defers API until the session state is not Connected or
  ReadOnlyConnected(ReadOnlyConnected only handles Read API. In other
  words, it also defers Write API).
  Because of it, ZooKeeperActor doesn't ensure order of API call. But it
  is not a problem, because ZooKeeper doesn't ensure it too. If you have
  to ensure order, use callback.

 2. Re-run command when timeout occured.

  ZooKeeper API user have to handle the situation that API calling is
  not transferred to the Server, because ZooKeeperActor is often
  disconnected.
  ZooKeeperActor has the requested command with CommandID and run
  scheduler per command. When timeout occured, ZooKeeperActor retrys the
  command if callback is not executed yet.

  CAUTION : Current implementation has a problem. Creation of ephemeral
  sequential node is executed multiple times, because it just ensures
  each command executed at least once, instead of executed only once.

ZooKeeperConnection is a dedicated actor for connection to ZooKeeper
Server. ZooKeeperActor creates it internally and restarts it when the
session is expired.