optimize: remove the branch registration operation of the XA read-only transaction

[GoodBoyCoder]

…n and other related operations

• I have registered the PR changes.

Ⅰ. Describe what this PR did

XA模式下。如果是只读事务就不注册分支及xa的任何操作
In XA mode, if it is a read-only transaction, the branch and any operation of xa will not be registered.

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

Manually set the connection's readOnly to true or set the annotation @transactional(readOnly = true) in the Spring framework

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 52.34%. Comparing base (be01fb0) to head (3abdbbe).
Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...ache/seata/rm/datasource/xa/ConnectionProxyXA.java	60.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #6826      +/-   ##
============================================
+ Coverage     52.30%   52.34%   +0.03%     
- Complexity     6405     6421      +16     
============================================
  Files          1083     1083              
  Lines         37846    37916      +70     
  Branches       4489     4495       +6     
============================================
+ Hits          19795    19846      +51     
- Misses        16081    16097      +16     
- Partials       1970     1973       +3     

Files with missing lines	Coverage Δ
...ache/seata/rm/datasource/xa/ConnectionProxyXA.java	56.97% <60.00%> (+1.94%)	⬆️

... and 6 files with indirect coverage changes

[funky-eyes]

LGTM

[xingfudeshi]

LGTM