make read_exact_at cancel-safe

[problame]

Before this PR, we didn't move ownership of the FileGuard to the read future.

This meant the read_exact_at future wasn't cancel-safe, because dropping it while the read future was live could lead to use-after-close of the file descriptor.
We protected against that through a scopeguard.

This PR builds on top of neondatabase/tokio-epoll-uring#27
, passing FileGuard ownership to the tokio-epoll-uring read() operation.

This makes the read_exact_at cancel-safe.

[github-actions]

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_{The comment gets automatically updated with the latest test results
53ac78e at 2023-12-12T14:21:15.476Z :recycle:}

[problame]

Compile error:

error: implementation of `std::marker::Send` is not general enough
  --> pageserver/src/consumption_metrics.rs:61:5
   |
61 | /     task_mgr::spawn(
62 | |         BACKGROUND_RUNTIME.handle(),
63 | |         TaskKind::CalculateSyntheticSize,
64 | |         None,
...  |
77 | |         },
78 | |     );
   | |_____^ implementation of `std::marker::Send` is not general enough
   |
   = note: `std::marker::Send` would have to be implemented for the type `&'0 VirtualFile`, for any lifetime `'0`...
   = note: ...but `std::marker::Send` is actually implemented for the type `&'1 VirtualFile`, for some specific lifetime `'1`

error: higher-ranked lifetime error
    --> pageserver/src/tenant/mgr.rs:2116:5
     |
2116 | /     task_mgr::spawn(
2117 | |         &tokio::runtime::Handle::current(),
2118 | |         TaskKind::GarbageCollector,
2119 | |         Some(tenant_id),
...    |
2164 | |         }
2165 | |     );
     | |_____^
     |
     = note: could not prove `{async block@pageserver/src/tenant/mgr.rs:2123:9: 2164:10}: std::marker::Send`

I think this is rust-lang/rust#102211 (comment) although I freely admit I don't fully understand that issue.

Anyway, following @jcsp 's advice I created a smaller reproducer and found that removing the FileGuard's lifetime generic parameter fixed the problem.

Follow the process in problame/integrate-tokio-epoll-uring/cancel-safe-read-exact-at--2023-12-11-fix-compile-error (the last two commits are interesting)