bugfix: new connection can't be get when connection is not released

[zjinlei]

Ⅰ. Describe what this PR did

When an rm is executed, its own business execution requires a database connection, Seata took 2 additional connections for the table meta.
During concurrency, there is not enough connection to the table meta, and druid will wait forever if it can't get the connection, so rm stuck.
The solution to this pr: makes connection reuse.

Ⅱ. Does this pull request fix one issue?

fixes #2069

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2068 into develop will decrease coverage by 0.1%.
The diff coverage is 40%.

@@              Coverage Diff              @@
##             develop    #2068      +/-   ##
=============================================
- Coverage      55.26%   55.16%   -0.11%     
+ Complexity      2523     2515       -8     
=============================================
  Files            446      446              
  Lines          14927    14922       -5     
  Branches        1770     1768       -2     
=============================================
- Hits            8249     8231      -18     
- Misses          5911     5923      +12     
- Partials         767      768       +1

Impacted Files	Coverage Δ	Complexity Δ
...ata/rm/datasource/undo/AbstractUndoLogManager.java	59.14% <0%> (ø)	17 <0> (ø)	⬇️
...n/java/io/seata/rm/datasource/DataSourceProxy.java	54.28% <0%> (-23.84%)	9 <0> (-2)
...o/seata/rm/datasource/AbstractConnectionProxy.java	11.7% <0%> (-5.51%)	5 <0> (-3)
...asource/sql/struct/cache/OracleTableMetaCache.java	6.12% <0%> (+0.24%)	3 <0> (ø)	⬇️
.../rm/datasource/exec/BaseTransactionalExecutor.java	44.35% <100%> (ø)	20 <0> (ø)	⬇️
...tasource/sql/struct/cache/MysqlTableMetaCache.java	81.13% <100%> (+0.05%)	11 <1> (ø)	⬇️
...ource/sql/struct/cache/AbstractTableMetaCache.java	81.81% <80%> (-6.07%)	9 <1> (-2)
...o/seata/rm/datasource/AbstractDataSourceProxy.java	23.07% <0%> (-7.7%)	1% <0%> (-1%)

[slievrly]

Most valuable PR.

[l81893521]

LGTM

[funky-eyes]

LGTM