Fine-tune the Ordering for num_senders

[wang384670111]

futures-rs/futures-channel/src/mpsc/mod.rs

Line 283 in aafe554

num_senders: AtomicUsize,

As mentioned in the comments, num_senders is used only for counting purposes, not for synchronizing access to other shared variables. While SeqCst guarantees the correctness of the program, it can also affect the performance. Therefore, using Relaxed is sufficient to maintain the correctness of the program without affecting its efficiency.

ed3b268 Could you please explain the reasoning behind the need to switch to seqcst?