storage: use object to pin and unpin version

[skyzh]

https://github.com/risinglightdb/risinglight/blob/7b5bc7a1a258afd64a483990ca95f4c799482791/src/storage/secondary/version_manager.rs#L258-L263

Currently, users will need to manually call pin and unpin. If there are futures cancelled or somehow, it is possible that we fail to unpin by ourselves.

We should implement a struct called Version, and implement Drop for it. pin will return a Arc<Version> instead of some strange things. When a Version gets dropped (which means that all reference of Arc<Version> is now dropped), we can call unpin in Drop.

[skyzh]

pub struct Version(u64, Arc<Snapshot>);

impl Drop for Version {
 // ...
}

pub fn pin(&self) -> Arc<Version> { /* .. */ }

[Yisaer]

I'm not familiar with rust, May I try this issue? :)

[skyzh]

Sure, thanks for that!