Add ZooKeeper data source for Sentinel

[guonanjun]

Describe what this PR does / why we need it

增加了zookeeper的数据源和测试demo

Resolves #29

Describe how you did it

参考nacos数据源，保持和nacos数据源风格一致，通过zkclient实现zookeeper的发布订阅功能，可以在规则变更的时候主动推送规则。

Describe how to verify it

sentinel-demo-zookeeper-datasource里面的ZookeeperDataSourceDemo是模拟微服务启动的时候注册数据源的场景，ZookeeperConfigSender是模拟Sentinel控制台在规则变更时，主动推送规则给微服务的场景。

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #33      +/-   ##
============================================
- Coverage     46.54%   46.37%   -0.17%     
+ Complexity      544      541       -3     
============================================
  Files           105      105              
  Lines          3657     3657              
  Branches        519      519              
============================================
- Hits           1702     1696       -6     
- Misses         1751     1754       +3     
- Partials        204      207       +3

Impacted Files	Coverage Δ	Complexity Δ
...ntinel/slots/statistic/metric/WindowLeapArray.java	70.96% <0%> (-9.68%)	7% <0%> (-1%)
...a/com/alibaba/csp/sentinel/node/StatisticNode.java	61.4% <0%> (-5.27%)	15% <0%> (-2%)

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[sczyh30]

Thanks for contribution! Maybe using Curator is better? It's considered that Curator is better supported than zkclient.

[guonanjun]

将zkclient替换为curator

[sczyh30]

Good. Maybe a basic test case is required since Curator has a local test server for testing (curator-test module). Nacos integration does not have test cases because Nacos does not provide this kind of mechanism currently.

[sczyh30]

The groupId and dataId are concepts in Nacos. In ZooKeeper a path is enough (should check to make sure it's started with /).

[guonanjun]

为了保持和nacos风格统一，groupId和dataId都不用以 / 开头，数据源里面会自动加。
String path = "/" + this.groupId + "/" + this.dataId;
Stat stat = this.zkClient.checkExists().forPath(path);

[sczyh30]

我感觉这里需要加一个只需传入 path 的构造函数：

public ZookeeperDataSource(final String serverAddr, final String path, ConfigParser<String, T> parser)

然后 groupId 和 dataId 版本的复用即可：

public ZookeeperDataSource(final String serverAddr, final String groupId, final String dataId, ConfigParser<String, T> parser) {
  this(serverAddr, getPath(groupId, dataId), parser);
}

因为对 ZooKeeper 用户来说，没有 groupId 和 dataId 的概念。即使为了平滑迁移到 Nacos 而保留这个版本的构造函数，也需要给用户最原始的方式指定 path。对迁移到 Nacos 来说也没有什么成本，封装一个 getPath 函数即可。

[sczyh30]

LGTM