#1432, move inner class CommandExecutor out for SkyWalking intercept

[tuohai666]

Fixes #1432 .

[coveralls]

Pull Request Test Coverage Report for Build 5568

• 2 of 45 (4.44%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.7%) to 68.134%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sharding-proxy/src/main/java/io/shardingsphere/shardingproxy/frontend/mysql/MySQLFrontendHandler.java	0	2	0.0%
sharding-proxy/src/main/java/io/shardingsphere/shardingproxy/frontend/mysql/CommandExecutor.java	2	43	4.65%

Files with Coverage Reduction	New Missed Lines	%
sharding-core/src/main/java/io/shardingsphere/core/parsing/antlr/parser/dialect/OracleStatementAdvancedParser.java	1	87.5%
sharding-core/src/main/java/io/shardingsphere/core/parsing/antlr/parser/dialect/PostgreStatementAdvancedParser.java	1	87.5%
sharding-core/src/main/java/io/shardingsphere/core/parsing/antlr/parser/dialect/MySQLStatementAdvancedParser.java	1	87.5%
sharding-core/src/main/java/io/shardingsphere/core/parsing/antlr/parser/dialect/SQLServerStatementAdvancedParser.java	1	87.5%

Totals
Change from base Build 5558:	0.7%
Covered Lines:	8174
Relevant Lines:	11997

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💛 - Coveralls