Fix the priority of cors and route

[xuchaoying]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests is needed?
  • Tests for the changes have been added
• Docs is needed?
  • Docs have been added / updated

What kind of change does this PR introduce?

• Bug fix
• Feature
• Enhencement
• Refactor
• Documents
• Others

1. Priority of cors should be higher than that of route. Add priorioty 300 to plugin cors

What is the related issue?

#172

Does this PR introduce a breaking change?

• breaking change？

Other information:

[codecov]

Codecov Report

Merging #173 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #173      +/-   ##
=========================================
+ Coverage    95.7%   95.7%   +<.01%     
=========================================
  Files          49      49              
  Lines        1675    1676       +1     
=========================================
+ Hits         1603    1604       +1     
  Misses         72      72

Impacted Files	Coverage Δ
packages/svrx/lib/constant.js	100% <ø> (ø)	⬆️
packages/svrx/lib/plugin/svrx-plugin-cors/index.js	87.5% <100%> (+1.78%)	⬆️

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