[Feature] Add CPU usage checking support in system protection rule

[CarpenterLee]

Describe what this PR does / why we need it

Sentinel System Guard takes average RT, QPS, thread count and system load into account, but does not consider CPU usage. Comparing to system load, CPU usage is more sensitive, which is useful when vast requests coming in sudden.

Does this pull request fix one issue?

Resolves #450

Describe how you did it

Add CPU usage threshold into system rule, and when current CPU usage reaches the threshold, a BBR likely checking will be done.

Describe how to verify it

Rule SystemGuardDemo.

[sczyh30]

As the OperatingSystemMXBean#getSystemCpuLoad API was introduced since JDK 1.7, the minimum JDK version of sentinel-core needs to be upgraded to 1.7 too (related to #235). This will bring breaking change so we're targeting this feature to next big milestone version (1.5.0).

[codecov-io]

Codecov Report

Merging #484 into master will increase coverage by 0.64%.
The diff coverage is 40.47%.

@@             Coverage Diff             @@
##             master    #484      +/-   ##
===========================================
+ Coverage     37.96%   38.6%   +0.64%     
- Complexity     1111    1160      +49     
===========================================
  Files           259     259              
  Lines          8177    8385     +208     
  Branches       1115    1176      +61     
===========================================
+ Hits           3104    3237     +133     
- Misses         4668    4737      +69     
- Partials        405     411       +6

Impacted Files	Coverage Δ	Complexity Δ
.../alibaba/csp/sentinel/slots/system/SystemRule.java	28.57% <25%> (-0.7%)	8 <1> (+1)
...a/csp/sentinel/slots/system/SystemRuleManager.java	43.79% <39.28%> (-1.82%)	8 <0> (ø)
...sp/sentinel/slots/system/SystemStatusListener.java	48.14% <66.66%> (+0.32%)	4 <0> (ø)	⬇️
.../sentinel/slots/statistic/data/ParamMapBucket.java	80.76% <0%> (-2.57%)	10% <0%> (ø)
.../src/main/java/com/alibaba/csp/sentinel/CtSph.java	73.84% <0%> (-0.39%)	40% <0%> (+16%)
.../handler/FetchClusterServerInfoCommandHandler.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...sentinel/cluster/flow/ClusterParamFlowChecker.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...tinel/slots/block/flow/param/ParamFlowChecker.java	47.41% <0%> (+0.42%)	29% <0%> (+9%)	⬆️
...slots/block/flow/controller/DefaultController.java	62.96% <0%> (+0.46%)	12% <0%> (+6%)	⬆️
...sentinel/eagleeye/EagleEyeRollingFileAppender.java	31.21% <0%> (+2.31%)	12% <0%> (+1%)	⬆️
... and 5 more

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[sczyh30]

LGTM

[sczyh30]

Thanks!