分库分表：Merger 抽象与批量查询实现

[codecov-commenter]

Codecov Report

Merging #160 (5a888b0) into dev (b22d20b) will decrease coverage by 0.10%. The diff coverage is 75.92%.

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@@            Coverage Diff             @@
##              dev     #160      +/-   ##
==========================================
- Coverage   79.82%   79.73%   -0.10%     
==========================================
  Files          28       29       +1     
  Lines        2275     2329      +54     
==========================================
+ Hits         1816     1857      +41     
- Misses        381      390       +9     
- Partials       78       82       +4     

Impacted Files	Coverage Δ
internal/merger/batchmerger/merger.go	75.92% <75.92%> (ø)

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