fixbug: should use db connection rather than tx connection exec sql r…

[dk-lockdown]

…equest

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 #8 (7873fad) into dev (045ab6a) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev       #8   +/-   ##
=======================================
  Coverage   57.34%   57.34%           
=======================================
  Files          19       19           
  Lines        1395     1395           
=======================================
  Hits          800      800           
  Misses        576      576           
  Partials       19       19           

Impacted Files	Coverage Δ
pkg/executor/read_write_splitting.go	0.00% <0.00%> (ø)

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