fix too many connection of zookeeper datasource

[linlinisme]

Describe what this PR does / why we need it

Fix "too many connections of zookeeper datasource"

Does this pull request fix one issue?

Related to #612

Describe how you did it

Save zkClient in a global map and reuse it

Describe how to verify it

1. 5 rules enabled
2. echo cons | nc zookeeper.host 2181
3. check if there only exists one connection to zkServer

Special notes for reviews

[codecov-io]

Codecov Report

Merging #788 into master will increase coverage by 0.18%.
The diff coverage is 83.78%.

@@             Coverage Diff              @@
##             master     #788      +/-   ##
============================================
+ Coverage     42.08%   42.27%   +0.18%     
- Complexity     1411     1420       +9     
============================================
  Files           307      307              
  Lines          8868     8897      +29     
  Branches       1200     1204       +4     
============================================
+ Hits           3732     3761      +29     
+ Misses         4679     4678       -1     
- Partials        457      458       +1

Impacted Files	Coverage Δ	Complexity Δ
...inel/datasource/zookeeper/ZookeeperDataSource.java	67.32% <83.78%> (+6.21%)	16 <6> (+7)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.29% <0%> (ø)	34% <0%> (ø)	⬇️
...tinel/slots/block/flow/param/ParamFlowChecker.java	55.4% <0%> (+2.7%)	29% <0%> (+1%)	⬆️
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	100% <0%> (+4.76%)	8% <0%> (+1%)	⬆️

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[sczyh30]

Friendly ping :)

[sczyh30]

Could you please add some basic test cases?

[linlinisme]

test cases had added

[linlinisme]

friendly ping :)

[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!