DataSource integration for Consul

[wavesZh]

Describe what this PR does / why we need it

add consul data source

Does this pull request fix one issue?

#870

Describe how you did it

The data source first initial rules from a Consul during initialization.
Then it start a watcher to observe the updates of rule date and update to memory.

Because Consul do not provide http api to watch the update of KV, so i use a long polling and
blocking queries of the Consul's feature to deal it. When Querying data by index will blocking until change or timeout. If the index of the current query is larger than before, it means that the data has changed.

Describe how to verify it

see UT

Special notes for reviews

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #979 into master will increase coverage by 0.21%.
The diff coverage is 61.9%.

@@             Coverage Diff              @@
##             master     #979      +/-   ##
============================================
+ Coverage     42.56%   42.77%   +0.21%     
- Complexity     1441     1450       +9     
============================================
  Files           310      311       +1     
  Lines          8984     9047      +63     
  Branches       1221     1228       +7     
============================================
+ Hits           3824     3870      +46     
- Misses         4693     4704      +11     
- Partials        467      473       +6

Impacted Files	Coverage Δ	Complexity Δ
...p/sentinel/datasource/consul/ConsulDataSource.java	61.9% <61.9%> (ø)	7 <7> (?)
...inel/datasource/zookeeper/ZookeeperDataSource.java	66.33% <0%> (-1%)	15% <0%> (-1%)
...tinel/slots/block/flow/param/ParamFlowChecker.java	55.4% <0%> (+2.7%)	29% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.29% <0%> (+2.97%)	34% <0%> (+1%)	⬆️
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	100% <0%> (+4.76%)	8% <0%> (+1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 17365f3...dd43513. Read the comment docs.

[sczyh30]

Could you please add a README.md in the module for some instructions? You may refer to: https://github.com/alibaba/Sentinel/blob/master/sentinel-extension/sentinel-datasource-redis/README.md

[wavesZh]

Could you please add a README.md in the module for some instructions? You may refer to: https://github.com/alibaba/Sentinel/blob/master/sentinel-extension/sentinel-datasource-redis/README.md

👌

[sczyh30]

And you could describe how the data source works in the PR description (e.g. it leveraged long polling).

[sczyh30]

LGTM

[sczyh30]

Nice, thanks for contributing!

[wavesZh]

Nice, thanks for contributing!

Thank you for your advice！