[ROCKETMQ-292]Delete system.exit in MQAdminStartup

[lindzh]

What is the purpose of the change

When using MQAdminStartup in a java process,some args problem may cause MQAdminStartup to call system.ext and this lead to main java process shutdown.
JIRA: https://issues.apache.org/jira/browse/ROCKETMQ-278.

Brief change log

• delete system.exit in mqadmin.

Verifying this change

• This change is a trivial rework / code cleanup without unit test coverage.
• execute mqadmin testa to verify.

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[coveralls]

Coverage increased (+0.02%) to 38.826% when pulling a905b14 on lindzh:fix_main_system.exit into 27a678d on apache:develop.

[vongosling]

Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add integration-test in test module.

Have you provide the test module ?

[vongosling]

System.exit() can be used to run shutdown hooks before the program quits. This is a convenient way to handle shutdown in bigger programs, where all parts of the program can't (and shouldn't) be aware of each other.
Another, maybe more common, way to quit a program is to simply to reach the end of the main method. But if there are any non-daemon threads running, they will not be shut down and thus the JVM will not exit. Thus, if you have any such non-daemon threads, you need some other means (than the shutdown hooks) to shut down all non-daemon threads and release other resources. If there are no other non-daemon threads, returning from main will shut down the JVM and will call the shutdown hooks.

For this reason, I am glad to see this improvement, thanks @lindzh

@shroman @Jaskey any opinion about this :-)

[lindzh]

I think there is no need to add any test case for this issue.Any opinion? IMO

[shroman]

I think we just need to check if there're no such non-daemon threads, and commit this pr ;)

[dongeforever]

LGTM

[Jaskey]

LGTM

[zhouxinyu]

LGTM~

[zhouxinyu]

merged!