Improve the checking logic of SystemRule and enhance SystemRuleManager

[sczyh30]

Describe what this PR does / why we need it

Improve SystemRuleManager and refactor the checking logic of maxCpuUsage strategy in SystemRule.

Does this pull request fix one issue?

NONE

Describe how you did it

• The BBR period for maxCpuUsage strategy is deprecated. Now Sentinel will just check the CPU usage value, which could avert surge of CPU usage quickly.
• Improve SystemRuleManager and SystemStatusListener.
• Add some test cases.

Describe how to verify it

Run the test cases.

Special notes for reviews

NONE

[codecov-io]

Codecov Report

Merging #1050 into master will increase coverage by 0.44%.
The diff coverage is 29.62%.

@@             Coverage Diff              @@
##             master    #1050      +/-   ##
============================================
+ Coverage     42.64%   43.08%   +0.44%     
- Complexity     1472     1493      +21     
============================================
  Files           317      317              
  Lines          9286     9290       +4     
  Branches       1267     1268       +1     
============================================
+ Hits           3960     4003      +43     
+ Misses         4833     4795      -38     
+ Partials        493      492       -1

Impacted Files	Coverage Δ	Complexity Δ
...sp/sentinel/slots/system/SystemStatusListener.java	37.03% <0%> (-4.63%)	3 <0> (+1)
...a/csp/sentinel/slots/system/SystemRuleManager.java	65% <66.66%> (+21.83%)	19 <2> (+12)	⬆️
...tinel/slots/block/flow/param/ParamFlowChecker.java	53.37% <0%> (-2.03%)	29% <0%> (ø)
...p/sentinel/datasource/consul/ConsulDataSource.java	63.76% <0%> (-1.45%)	7% <0%> (ø)
...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%)	⬆️
.../alibaba/csp/sentinel/slots/system/SystemRule.java	44.89% <0%> (+16.32%)	12% <0%> (+4%)	⬆️
...sp/sentinel/slots/system/SystemBlockException.java	44.44% <0%> (+44.44%)	2% <0%> (+2%)	⬆️

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[ghost]

LGTM

[fangwentong]

The BBR period for maxCpuUsage strategy is deprecated. Now Sentinel will just check the CPU usage value, which could avert surge of CPU usage quickly.

CPU usage value updates every seconds, this change means that if CPU usage exceed threshold, all the traffic in next second will be blocked?