[ROCKETMQ-320] Message loss when shutdown with dispatch behind

[dongeforever]

What is the purpose of the change

Avoid message loss when shutdown with dispatch behind

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[coveralls]

Coverage increased (+0.04%) to 39.171% when pulling c572d03 on dongeforever:ROCKETMQ-320 into 4926187 on apache:develop.

[zhouxinyu]

LGTM~

[vongosling]

+1, Cool~

[shroman]

+1
Why do we need shutDownNormal here?

[zhouxinyu]

@shroman It's only for unit test, any better solution?

[shroman]

@zhouxinyu I would throw an exception from DefaultMessageStore#shutdown if something goes wrong. This will also simplify unit tests -- no special flags are needed.