fix: change the event type that SqlSessionFactory is listening to

[SkyAndCode]

fixes #796

This is why I need to change it.

1. From the implementation of SqlSessionFactory, I think that it does not need to listen to all ApplicationEvents, and only needs to listen to ContextRefreshedEvent.
2. When there are multiple ApplicationContexts in Spring, ContextClosedEvent may be broadcasted multiple times. For example, FeignContext, since SqlSessionFactory listens to all ApplicationEvents, if the application is closed, when SqlSessionFactoryBean is destroyed first, and FeignContext is destroyed later, ContextClosedEvent will be broadcasted. When Spring tries to get the listener, BeanCreationNotAllowedException is thrown because SqlSessionFactoryBean's singleton bean has been destroyed and it indicates that singletons are currently being destroyed (singletonsCurrentlyInDestruction=true).
   Although Spring considers this only a warning, I think that SqlSessionFactoryBean does not need to listen to all ApplicationEvents, and this warning should not appear in this case.

[hazendaz]

@kazuki43zoo This seems logical given the override only checks ContextRefreshedEvent before doing anything, your thoughts?

[coveralls]

Coverage: 89.392%. Remained the same when pulling e8a2a57 on SkyAndCode:change-sqlsessionfactory-event-type into 531bd26 on mybatis:master.

[hazendaz]

no feedback, I'm pretty sure this is good, merging.