[PG Move]destroy PG needs to be async

[yamingk]

During PG Move on receipt of on_destroy, it needs to support async destroy of index table, because on_destory happens in the context of nuraft message state machine, making it async will unblock the state machine while destroying the btree.

[JacksonYao287]

@yuwmao can we close this? IIRC , we will pause state_machine before destroy pg

[yuwmao]

Yes, we will pause the state_machine before destroy pg.
Sorry, I don't understand the problem well. Why does blocking statemachine matter since it already going to be destroyed. And in my understand, the subsequent logs won't be sent to this member.