fix #662 repeated loading DataSourceManager

[github-ygy]

Ⅰ. Describe what this PR did

fix repeated loading DataSourceManager

Ⅱ. Does this pull request fix one issue?

#662 #667

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #750 into develop will increase coverage by 0.09%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop     #750      +/-   ##
=============================================
+ Coverage      32.31%   32.41%   +0.09%     
  Complexity       928      928              
=============================================
  Files            230      228       -2     
  Lines           9088     9060      -28     
  Branches        1103     1103              
=============================================
  Hits            2937     2937              
+ Misses          5804     5776      -28     
  Partials         347      347

Impacted Files	Coverage Δ	Complexity Δ
...libaba/fescar/rm/datasource/DataSourceManager.java	0% <ø> (ø)	0 <0> (ø)	⬇️
.../com/alibaba/fescar/rm/datasource/AsyncWorker.java	0% <0%> (ø)	0 <0> (ø)	⬇️

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[xingfudeshi]

LGTM

[slievrly]

LGTM