Make SentinelConfig.loadProps thread safe

[jasonjoo2010]

Describe what this PR does / why we need it

Make SentinelConfig.loadProps thread safe

Does this pull request fix one issue?

Related to #705

Describe how you did it

Make snapshot of System.properties

Describe how to verify it

public class DashboardApplication {

    public static void main(String[] args) {
        new Thread() {
            public void run() {
                for (int i = 0; i < 10000; i ++) {
                    System.setProperty("prefix-" + i, "test");
                    try {
                        sleep(1);
                    } catch (InterruptedException e) {
                    }
                }
            }
        }.start();
        triggerSentinelInit();
        SpringApplication.run(DashboardApplication.class, args);
    }

    private static void triggerSentinelInit() {
        new Thread(() -> InitExecutor.doInit()).start();
    }
}

[codecov-io]

Codecov Report

Merging #706 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master    #706   +/-   ##
========================================
  Coverage      40.4%   40.4%           
+ Complexity     1312    1311    -1     
========================================
  Files           297     297           
  Lines          8591    8591           
  Branches       1156    1156           
========================================
  Hits           3471    3471           
  Misses         4695    4695           
  Partials        425     425

Impacted Files	Coverage Δ	Complexity Δ
...om/alibaba/csp/sentinel/config/SentinelConfig.java	51.61% <100%> (ø)	14 <0> (ø)	⬇️
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	95.23% <0%> (-4.77%)	7% <0%> (-1%)
...m/alibaba/csp/sentinel/log/DateFileLogHandler.java	54.54% <0%> (-3.04%)	6% <0%> (-1%)
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.58% <0%> (+2.94%)	34% <0%> (+1%)	⬆️

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[sczyh30]

LGTM

[sczyh30]

Thanks!