modify MethodUtil#methodNameMap and CtSph#chainMap, add volatile.

[manzhizhen]

…p in line with

Describe what this PR does / why we need it

modify MethodUtil#methodNameMap and CtSph#chainMap, add volatile, keep in line with ContextUtil#contextNameNodeMap.

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

Special notes for reviews

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #34 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #34      +/-   ##
============================================
- 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
...java/com/alibaba/csp/sentinel/util/MethodUtil.java	76.92% <100%> (ø)	3 <1> (ø)	⬇️
.../src/main/java/com/alibaba/csp/sentinel/CtSph.java	65.9% <100%> (ø)	8 <0> (ø)	⬇️
...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]

LGTM

[sczyh30]

Thanks for your contribution. Could you please sign the CLA here: https://cla-assistant.io/alibaba/Sentinel?pullRequest=34