reactor: select for update executor

[Charlie17Li]

Signed-off-by: charlie qianglin98@qq.com

What this PR does:

1. reactor the "select for update executor"
2. fix the unit test for "select for update executor" failed

Which issue(s) this PR fixes:

Fixes #442

ref pr #465

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #478 (af122b7) into master (437f712) will decrease coverage by 0.08%.
The diff coverage is 29.46%.

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@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
- Coverage   39.05%   38.98%   -0.08%     
==========================================
  Files         144      146       +2     
  Lines        9120     9342     +222     
==========================================
+ Hits         3562     3642      +80     
- Misses       5264     5403     +139     
- Partials      294      297       +3     

Impacted Files	Coverage Δ
pkg/client/client.go	0.00% <0.00%> (ø)
pkg/datasource/sql/exec/at/at_executor.go	0.00% <0.00%> (ø)
pkg/datasource/sql/exec/at/config.go	0.00% <0.00%> (ø)
pkg/datasource/sql/exec/at/delete_executor.go	22.77% <0.00%> (-0.70%)	⬇️
.../datasource/sql/exec/select_for_update_executor.go	0.00% <0.00%> (ø)
pkg/datasource/sql/types/executor.go	0.00% <0.00%> (ø)
.../sql/undo/builder/mysql_update_undo_log_builder.go	0.00% <0.00%> (ø)
...tasource/sql/exec/at/select_for_update_executor.go	29.95% <29.95%> (ø)
pkg/datasource/sql/exec/at/update_executor.go	25.00% <33.33%> (ø)
pkg/datasource/sql/exec/at/insert_executor.go	41.40% <57.14%> (+0.03%)	⬆️
... and 3 more

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