Move Zookeeper Access Layer into separate module

[kaisun2000]

First step in horizontally scaling Zookeeper in Helix. Consolidate the Zookeeper accessing code into a separate module, helix-zk.

The intended class and interfaces to be moved are the following:
HelixZkClient
HelixZkClientFactory
DedicatedZkClientFactory
SharedZkClientFactory
ZkConnectionManager

zookeeper/ZkClient
zookeeper/ZkConnection
zookeeper/ZkEventThread

PathBasedZkSerializer
BasicZkSerializer
ChainedPathZkSerializer
PreFetch
ZkAsyncCallbacks
ZkClient

The goal is to make it backward compatible. Namely users of the helix-core would not need to change code that they would still be able to compile the code and maintain the same behavior

[narendly]

Do we still want to call it "helix-zk"? I thought the idea was to have a module fully decoupled from Helix's logic. In that case, names like "zookeeper-core" or "zk-api" might be more appropriate?

Also when you create an issue, please state the "why" in your description.

@lei-xia

[kaisun2000]

Will share the diff soon. Feel free to comment on the naming there.