fix: ssr perf

[ycjcl868]

Close umijs/umi-server#35.

• getInitialProps 函数内建议使用 return，不然会直接返回整个 model
• 优化 serialize-javascript ，只 require 一次。

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

[label-actions]

@ycjcl868 请看下。

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[codecov]

Codecov Report

Merging #3696 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #3696   +/-   ##
=======================================
  Coverage   40.38%   40.38%           
=======================================
  Files         283      283           
  Lines        7956     7956           
  Branches     1865     1915   +50     
=======================================
  Hits         3213     3213           
+ Misses       4366     4360    -6     
- Partials      377      383    +6

Impacted Files	Coverage Δ
packages/umi-build-dev/src/FilesGenerator.js	1.5% <ø> (ø)	⬆️
packages/umi-plugin-react/src/plugins/antd.js	0% <0%> (ø)	⬆️
...kages/umi-plugin-ui/src/plugins/dashboard/index.ts	0% <0%> (ø)	⬆️
...s/umi-build-dev/src/plugins/commands/block/util.ts	33.11% <0%> (ø)	⬆️
packages/umi-plugin-ui/src/bubble/index.jsx	0% <0%> (ø)	⬆️

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