optimize: Flatten Saga nested transactions

[long187]

Ⅰ. Describe what this PR did

Saga nested transaction(SubState Machine)will create a new Global transaction, it is not necessary, We need make it Flatten.

Ⅱ. Does this pull request fix one issue?

#1974

Ⅳ. Describe how to verify it

run unit test at seata/test/src/test/java/io/seata/saga

[codecov-io]

Codecov Report

Merging #1975 into develop will increase coverage by <.01%.
The diff coverage is 0%.

@@              Coverage Diff              @@
##             develop    #1975      +/-   ##
=============================================
+ Coverage      55.31%   55.32%   +<.01%     
+ Complexity      2439     2437       -2     
=============================================
  Files            434      434              
  Lines          14469    14470       +1     
  Branches        1706     1707       +1     
=============================================
+ Hits            8004     8005       +1     
+ Misses          5743     5742       -1     
- Partials         722      723       +1

Impacted Files	Coverage Δ	Complexity Δ
...o/seata/saga/statelang/domain/DomainConstants.java	0% <ø> (ø)	0 <0> (ø)	⬇️
.../statelang/parser/impl/StateMachineParserImpl.java	70.73% <0%> (-1.77%)	8 <0> (ø)
...in/java/io/seata/server/session/GlobalSession.java	84.54% <0%> (-0.49%)	67% <0%> (-1%)
...o/seata/server/coordinator/DefaultCoordinator.java	48.01% <0%> (-0.4%)	27% <0%> (-1%)
...server/store/file/FileTransactionStoreManager.java	57.46% <0%> (+0.95%)	29% <0%> (ø)	⬇️

[slievrly]

LGTM

[zjinlei]

LGTM

CI failed

[zjinlei]

LGTM