perf: optimize the performance of XA transactions

[iSuperCoder]

What this PR does:

取消了每次执行XA事务时查询数据库版本的操作，把版本查询放在了getOpenConnectorProxy过程中，只需要查一次即可。

Which issue(s) this PR fixes:
Fixes #553

[codecov-commenter]

Codecov Report

Merging #554 (37e97d8) into master (d62b776) will increase coverage by 0.35%.
The diff coverage is 69.23%.

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@@            Coverage Diff             @@
##           master     #554      +/-   ##
==========================================
+ Coverage   36.64%   36.99%   +0.35%     
==========================================
  Files         160      160              
  Lines       11138    11154      +16     
==========================================
+ Hits         4081     4126      +45     
+ Misses       6710     6681      -29     
  Partials      347      347              

Impacted Files	Coverage Δ
pkg/datasource/sql/connector.go	85.71% <ø> (+21.00%)	⬆️
pkg/rm/tcc/fence/fence_driver_tx.go	0.00% <ø> (ø)
pkg/datasource/sql/driver.go	69.71% <61.90%> (+3.38%)	⬆️
pkg/datasource/sql/db.go	52.03% <100.00%> (+7.78%)	⬆️

... and 1 file with indirect coverage changes

[iSuperCoder]

使用QueryContext在sample中测试select for update，加上def rows.Close()就没有busy buffer，去掉defer rows.Close就必现busy buffer异常。使用ExecContext测试没有这个问题。
我目前还没有定位到根本原因，可以继续看看，如果确实存在未解决的bug，可以考虑在新的PR中解决。当前PR仅优化数据库版本查询导致的XA事务耗时增加。