Add BatchSemaphore::upgrade and parking_lot::RwLock::upgrade

[sarsko]

Adds upgradeable reads to parking_lot::RwLock

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[sarsko]

Code is wrong, ref https://docs.rs/parking_lot/latest/parking_lot/type.RwLock.html#method.upgradable_read:

Locks this rwlock with upgradable read access, blocking the current thread until it can be acquired.

The calling thread will be blocked until there are no more writers **or other upgradable reads** which hold the lock. There may be other readers currently inside the lock when this method returns.

Returns an RAII guard which will release this thread's shared access once it is dropped.

I put **s around the important part, which is the other upgradeable reads part. Credit to this Reddit post: https://www.reddit.com/r/rust/comments/nbrh9v/upgradable_parking_lotrwlock_might_not_do_what/ and this article: https://morestina.net/1739/upgradable-parking_lotrwlock-might-not-be-what-you-expect for pointing this out.

[sarsko]

Made it match the semantics of the one from parking_lot (sans the fairness discussed in #259)

Playground I used to check behavior of the parking_lot RwLock:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=cffda8f89a8d2fd2dfe0f2f17fc5012b

[sarsko]

There is an alternative design which implements this with only one semaphore by making write map to upgradable_read + upgrade. That causes extra scheduling points for regular write acquires, and also makes it not atomic.