Remove atomic64 feature and use crossbeam::atomic::AtomicCell<u64> for all targets?

[tatsuya6502]

In order to support target platforms without AtomicU64, we added a Cargo feature called atomic64 to Moka v0.5.3 and put it into the default feature list (#38, #39).

• If the feature is enabled, Moka keeps the old behavior (by using AtomicU64 as timestamps of cache entries).
• Otherwise, it uses RwLock<Instant> so that it will compile for such targets.

The feature also controls whether quanta crate is used or not by Moka, because quanta v0.9.2 or earlier do not compile for such targets.

We might want to replace above with crossbeam::atomic::AtomicCell<u64> because it provides a similar functionality to AtomicU64 (when it is supported by the target) and RwLock<u64> (when AtomicU64 is not supported).

https://docs.rs/crossbeam/0.8.1/crossbeam/atomic/struct.AtomicCell.html

Operations on AtomicCells use atomic instructions whenever possible, and synchronize using global locks otherwise.

It will be more convenient as it switches implementations without having a Cargo feature like atomic64. It combines auto-generated Rust source and the build script to achieve auto-switching.

Also, the next version of quanta will support these targets (metrics-rs/quanta#55), so once it is released, we can always enable quanta.

[tatsuya6502]

Before switching to AtomicCell<T>, we need to check its implementation to see if it as efficient as our current implementation.

[tatsuya6502]

Also, the next version of quanta will support these targets (metrics-rs/quanta#55), so once it is released, we can always enable quanta.

quanta v0.9.3 has been released and Moka v0.5.4 (next version) and newer will always enable quanta (#42).

[tatsuya6502]

Started to use AtomicCell<u64> for wight based eviction: 4404e6f