server: optimize store topology weight memory

[rleungx]

What problem does this PR solve?

Issue Number: Close #9288.

What is changed and how does it work?

$ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: github.com/tikv/pd/server/cluster
cpu: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz
                                   │   old.txt    │               new.txt               │
                                   │    sec/op    │    sec/op     vs base               │
BuildTopology/Small-10-Stores-72     7.234µ ± 10%   8.568µ ± 34%        ~ (p=0.240 n=6)
BuildTopology/Medium-100-Stores-72   53.34µ ±  9%   67.69µ ± 15%  +26.91% (p=0.004 n=6)
BuildTopology/Large-1000-Stores-72   396.2µ ±  3%   466.3µ ±  5%  +17.69% (p=0.002 n=6)
geomean                              53.47µ         64.67µ        +20.95%

                                   │    old.txt     │                  new.txt                  │
                                   │      B/op      │     B/op       vs base                    │
BuildTopology/Small-10-Stores-72       5.867Ki ± 0%   0.000Ki ±  0%  -100.00% (p=0.002 n=6)
BuildTopology/Medium-100-Stores-72   19208.000 ± 0%     2.000 ± 50%   -99.99% (p=0.002 n=6)
BuildTopology/Large-1000-Stores-72    91449.00 ± 0%     16.00 ± 12%   -99.98% (p=0.002 n=6)
geomean                                21.42Ki                       ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

                                   │   old.txt   │                 new.txt                 │
                                   │  allocs/op  │  allocs/op   vs base                    │
BuildTopology/Small-10-Stores-72      79.00 ± 0%     0.00 ± 0%  -100.00% (p=0.002 n=6)
BuildTopology/Medium-100-Stores-72    539.0 ± 0%      0.0 ± 0%  -100.00% (p=0.002 n=6)
BuildTopology/Large-1000-Stores-72   4.144k ± 0%   0.000k ± 0%  -100.00% (p=0.002 n=6)
geomean                               560.9                     ?                      ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

Check List

Tests

• Unit test

Release note

None.

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Codecov Report

:x: Patch coverage is 92.78351% with 7 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 76.05%. Comparing base (7c3b9a3) to head (a367c94). :warning: Report is 273 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9293      +/-   ##
==========================================
+ Coverage   75.96%   76.05%   +0.08%     
==========================================
  Files         472      472              
  Lines       73249    73333      +84     
==========================================
+ Hits        55646    55771     +125     
+ Misses      14131    14094      -37     
+ Partials     3472     3468       -4     

Flag	Coverage Δ
unittests	76.05% <92.78%> (+0.08%)	:arrow_up:

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@rleungx: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-unit-test-next-gen-1	a367c9410499edb34d5fa4c1628f4c02eaf2f646	link	true	/test pull-unit-test-next-gen-1
pull-unit-test-next-gen-2	a367c9410499edb34d5fa4c1628f4c02eaf2f646	link	true	/test pull-unit-test-next-gen-2
pull-unit-test-next-gen-3	a367c9410499edb34d5fa4c1628f4c02eaf2f646	link	true	/test pull-unit-test-next-gen-3

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