src/bandwidth: Drop atomic dependency

[hrxi]

Replace atomic::Atomic<u64> by std::sync::atomic:AtomicU64.

The original motivation of using atomic::Atomic<u64> instead of
std's version in #1670 was the following:

I used the atomic crate and an Atomic because the AtomicU64 type
isn't available on all architectures. The atomic crate automatically
falls back to using a Mutex on platforms that don't support AtomicU64.

This argumentation is moot because the crate directly depends on
libp2p-core which also uses AtomicU64.

[thomaseizinger]

Thanks for the contribution!

1. Can you write a few lines about this change? i.e. is the atomic crate fully superseded by the std lib?
2. We don't have a lockfile checked into the repository because we are only developing a library here. Can you post the output of cargo tree -i -p atomic to show that we are actually dropping the dependency?
3. Rustfmt doesn't seem to be happy with the changes, please fix that :)

[hrxi]

1. Can you write a few lines about this change? i.e. is the atomic crate fully superseded by the std lib?

Done.

2. We don't have a lockfile checked into the repository because we are only developing a library here. Can you post the output of cargo tree -i -p atomic to show that we are actually dropping the dependency?

$ git checkout pr_drop_atomic
Previous HEAD position was f04f6bb4 identify/handler: Improve property name (#2639)
Switched to branch 'pr_drop_atomic'
$ cargo tree -i -p atomic
error: package ID specification `atomic` did not match any packages

	Did you mean `tokio`?
$ git checkout HEAD~
HEAD is now at f04f6bb4 identify/handler: Improve property name (#2639)
$ cargo tree -i -p atomic | head
atomic v0.5.1
├── libp2p v0.45.0 (/home/u/nimiq/rust-libp2p)
│   [dev-dependencies]
│   ├── libp2p-autonat v0.4.0 (/home/u/nimiq/rust-libp2p/protocols/autonat)
│   │   └── libp2p v0.45.0 (/home/u/nimiq/rust-libp2p) (*)
│   ├── libp2p-dcutr v0.3.0 (/home/u/nimiq/rust-libp2p/protocols/dcutr)
│   │   ├── libp2p v0.45.0 (/home/u/nimiq/rust-libp2p) (*)
│   │   └── libp2p-metrics v0.6.0 (/home/u/nimiq/rust-libp2p/misc/metrics)
│   │       └── libp2p v0.45.0 (/home/u/nimiq/rust-libp2p) (*)
│   ├── libp2p-mdns v0.37.0 (/home/u/nimiq/rust-libp2p/protocols/mdns)

3. Rustfmt doesn't seem to be happy with the changes, please fix that :)

Done.

[mxinden]

Thanks @hrxi!