Consider adding FuturesUnordered::clear() and SelectAll::clear()

[erickt]

We're using FuturesUnordered to race multiple connection futures in order to implement the RFC 8305 Happy Eyeballs spec. Once we make a successful connection, we're supposed to close all the other pending connections in order to free up the resources. We are wrapping our FuturesUnordered in an Option to make sure we can drop any other pending connections as soon as possible.

However, it be simpler if we could just call a FuturesUnordered::clear() to throw away the futures. Likewise, I imagine a SelectAll::clear() could also be useful.

[taiki-e]

This seems reasonable to me. I would accept a PR to add this.

[erickt]

Thanks! However @ibraheemdev ’s #2415 only implements clear for FuturesUnordered, not SelectAll. Would it be possible to implement clear for it as well?

[ibraheemdev]

ah, yes, that is an easy delegation. I'll open a PR.