optimize: opt ContextCore, can be set 'Object' value

[wangliang181230]

Ⅰ. Describe what this PR did

optimize: opt ContextCore, can be set 'Object' value.
优化ContextCore，可设置Object值。

优化的目的：

RootContext.getBranchType()和RootContext.unbindBranchType()方法的返回类型由String变为BranchType，部分判断方法的性能会更高，代码也会更简洁易读。

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

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #3102 into develop will decrease coverage by 0.00%.
The diff coverage is 56.66%.

@@              Coverage Diff              @@
##             develop    #3102      +/-   ##
=============================================
- Coverage      50.56%   50.55%   -0.01%     
+ Complexity      3103     3100       -3     
=============================================
  Files            599      599              
  Lines          19515    19516       +1     
  Branches        2404     2405       +1     
=============================================
- Hits            9867     9866       -1     
  Misses          8657     8657              
- Partials         991      993       +2     

Impacted Files	Coverage Δ	Complexity Δ
...o/seata/rm/datasource/AbstractConnectionProxy.java	11.34% <0.00%> (ø)	5.00 <0.00> (ø)
...a/io/seata/rm/datasource/exec/ExecuteTemplate.java	6.45% <0.00%> (ø)	2.00 <1.00> (ø)
...java/io/seata/spring/tcc/TccActionInterceptor.java	11.36% <0.00%> (ø)	2.00 <0.00> (ø)
...c/main/java/io/seata/core/context/RootContext.java	55.55% <60.00%> (+1.01%)	13.00 <4.00> (ø)
...baba/AlibabaDubboTransactionPropagationFilter.java	52.38% <80.00%> (ø)	8.00 <0.00> (ø)
...dubbo/ApacheDubboTransactionPropagationFilter.java	55.00% <80.00%> (ø)	8.00 <0.00> (ø)
...seata/core/context/FastThreadLocalContextCore.java	85.71% <100.00%> (ø)	4.00 <0.00> (ø)
.../io/seata/core/context/ThreadLocalContextCore.java	33.33% <100.00%> (ø)	1.00 <0.00> (ø)
...o/seata/server/coordinator/DefaultCoordinator.java	54.63% <0.00%> (-0.52%)	28.00% <0.00%> (-2.00%)
...in/java/io/seata/server/session/GlobalSession.java	82.43% <0.00%> (-0.46%)	70.00% <0.00%> (-1.00%)

[slievrly]

LGTM

[funky-eyes]

LGTM