rows: 同库事务语句合并执行，提前读取所有数据

[codecov[bot]]

Codecov Report

Merging #219 (ce0c771) into dev (07dc416) will decrease coverage by 0.34%. The diff coverage is 46.49%.

@@            Coverage Diff             @@
##              dev     #219      +/-   ##
==========================================
- Coverage   83.12%   82.78%   -0.34%     
==========================================
  Files          50       52       +2     
  Lines        3893     3951      +58     
==========================================
+ Hits         3236     3271      +35     
- Misses        540      563      +23     
  Partials      117      117              

Files	Coverage Δ
db.go	76.78% <100.00%> (ø)
...nternal/merger/groupby_merger/aggregator_merger.go	81.28% <100.00%> (ø)
internal/test/types.go	27.27% <ø> (ø)
sharding_select.go	72.45% <100.00%> (+11.86%)	:arrow_up:
session.go	95.00% <95.00%> (ø)
internal/rows/data_rows.go	94.59% <94.59%> (ø)
internal/rows/convert_assign.go	46.66% <16.66%> (-20.00%)	:arrow_down:
transaction.go	5.00% <0.00%> (-95.00%)	:arrow_down:

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