feat: segment sequence support define initial value

[dk-lockdown]

ref: https://github.com/cectc/dbpack/issues/

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

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

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #281 (40eac2f) into dev (1d12859) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head 40eac2f differs from pull request most recent head cea1d54. Consider uploading reports for the commit cea1d54 to get more accurate results

@@            Coverage Diff             @@
##              dev     #281      +/-   ##
==========================================
- Coverage   42.54%   42.52%   -0.02%     
==========================================
  Files          91       91              
  Lines       10470    10470              
==========================================
- Hits         4454     4452       -2     
- Misses       5527     5528       +1     
- Partials      489      490       +1     

Impacted Files	Coverage Δ
pkg/sequence/sequence.go	0.00% <0.00%> (ø)
pkg/sequence/segment.go	56.33% <66.66%> (ø)
third_party/pools/numbered.go	86.73% <0.00%> (-2.05%)	⬇️

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