(WIP)refactor:seata conn

[chuntaojun]

What this PR does:

// 待补充 tx 的单元测试

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #287 (8a60797) into master (94225ea) will increase coverage by 5.16%. The diff coverage is 57.31%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   50.99%   56.15%   +5.16%     
==========================================
  Files          61       67       +6     
  Lines        3106     3540     +434     
==========================================
+ Hits         1584     1988     +404     
+ Misses       1445     1431      -14     
- Partials       77      121      +44     

Impacted Files	Coverage Δ
pkg/datasource/sql/stmt.go	0.00% <0.00%> (ø)
pkg/datasource/sql/conn.go	23.96% <32.81%> (+23.96%)	:arrow_up:
pkg/datasource/sql/connector.go	40.47% <46.87%> (+40.47%)	:arrow_up:
pkg/datasource/sql/conn_at.go	57.89% <57.89%> (ø)
pkg/datasource/sql/conn_xa.go	57.89% <57.89%> (ø)
pkg/datasource/sql/driver.go	59.70% <82.92%> (+55.81%)	:arrow_up:
pkg/datasource/sql/tx.go	48.95% <92.59%> (+48.95%)	:arrow_up:
pkg/datasource/sql/context.go	100.00% <100.00%> (ø)
pkg/datasource/sql/exec/hook/basic_undo_builder.go	34.17% <0.00%> (-38.80%)	:arrow_down:
pkg/rm/tcc/tcc_resource.go	32.43% <0.00%> (-5.47%)	:arrow_down:
... and 13 more

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