rows, merger: 使用 sqlx.Rows 作为接口，并重构 merger 包

[codecov[bot]]

Codecov Report

Merging #217 (2716109) into dev (d918cee) will decrease coverage by 2.54%. Report is 2 commits behind head on dev. The diff coverage is 56.76%.

@@            Coverage Diff             @@
##              dev     #217      +/-   ##
==========================================
- Coverage   85.65%   83.12%   -2.54%     
==========================================
  Files          48       50       +2     
  Lines        3653     3893     +240     
==========================================
+ Hits         3129     3236     +107     
- Misses        415      540     +125     
- Partials      109      117       +8     

Files Changed	Coverage Δ
internal/rows/convert_assign.go	66.66% <ø> (ø)
sharding_select.go	60.58% <10.29%> (-13.06%)	:arrow_down:
internal/datasource/single/db.go	55.00% <28.57%> (-16.43%)	:arrow_down:
internal/merger/batchmerger/merger.go	87.09% <37.50%> (-7.45%)	:arrow_down:
...l/datasource/shardingsource/sharding_datasource.go	56.52% <40.74%> (-20.41%)	:arrow_down:
internal/datasource/cluster/cluster_db.go	60.00% <45.45%> (-21.82%)	:arrow_down:
...nternal/merger/groupby_merger/aggregator_merger.go	81.28% <64.28%> (-2.33%)	:arrow_down:
internal/merger/pagedmerger/merger.go	94.56% <66.66%> (-2.03%)	:arrow_down:
internal/merger/aggregatemerger/merger.go	91.72% <68.75%> (-1.80%)	:arrow_down:
...ernal/datasource/transaction/single_transaction.go	73.33% <73.33%> (ø)
... and 4 more

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more