Use AtomicU64 for epochs in 32-bit architectures

[jeehoonkang]

@stjepang said:

Just so we don't forget, in all the other places where we have atomic timestamps, we should use AtomicU64 (I believe we can't use wide sequences like in this PR, unfortunately):

• epoch representation
• head/tail index in deque, channel, and queues

But that can come later in another PR. It will also require Rust 1.34 minimum.

[jeehoonkang]

Since MSRV = 1.36 now, we can use AtomicU64 now: https://doc.rust-lang.org/std/sync/atomic/struct.AtomicU64.html

[taiki-e]

Note that some 32-bit architectures do not support AtomicU64: tokio-rs/tokio#1421 (comment)

[jeehoonkang]

Very sad. Do you think it's worth it to use AtomicU64 for those platforms that support it? My opinion is yes, because, e.g., major 32-bit architectures support AtomicU64.

[taiki-e]

I think it's good to use it when AtomicU64 is available, but we have to decide what to do if AtomicU64 is not available.
We can probably use AtomicCell<u64> for this, but performance may be degraded on platforms that AtomicU64 is not available.