Remove spin loop in MPSC channel

[albertlarsan68]

Separately, I think the retry loop when sending a message should be removed (like we did with the similar loop in recv). The reasoning is the same: we don't want to waste CPU cycles spinning when trying to write to a full channel. Instead we should just go ahead and block immediately. But this can be a separate PR.

Originally posted by @Amanieu in #106701 (comment)

This issue has been assigned to @ibraheemdev via this comment.

[albertlarsan68]

@rustbot claim

[marmeladema]

Done in #106836 I believe

[albertlarsan68]

@rustbot assign @ibraheemdev