Refactor XA-Transaction & Using ShardingTransactionEngine for proxy.

[cherrylzhao]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 6371

• 33 of 37 (89.19%) changed or added relevant lines in 9 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.007%) to 76.226%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sharding-transaction/sharding-transaction-2pc/sharding-transaction-xa/sharding-transaction-xa-core/src/main/java/io/shardingsphere/transaction/xa/jta/datasource/SingleXADataSource.java	4	5	80.0%
sharding-proxy/src/main/java/io/shardingsphere/shardingproxy/backend/jdbc/datasource/JDBCBackendDataSource.java	7	10	70.0%

Files with Coverage Reduction	New Missed Lines	%
sharding-proxy/src/main/java/io/shardingsphere/shardingproxy/backend/jdbc/datasource/JDBCBackendDataSource.java	3	51.02%

Totals
Change from base Build 6370:	0.007%
Covered Lines:	10029
Relevant Lines:	13157

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💛 - Coveralls