use rcu in DiskManager

[liwenhui-soul]

What type of PR is this?

• bug
• feature
• enhancement

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• Performance impacted: Consumes more CPU/Memory

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[critical27]

Maybe you don't need to declare a rcu_domain explicitly, the default one is enough.

[liwenhui-soul]

Maybe you don't need to declare a rcu_domain explicitly, the default one is enough.

metaclient also use default, they share the same one?

[critical27]

Good job~

[cangfengzhs]

LGTM