add schedule replace/cancel to allow creation of watchdog tasks

[ssendev]

maybe as spawn.replace.name(cycles: u32) and spawn.cancel.name() though that pollutes the task namespace so maybe respawn.name(cycles: u32)

[korken89]

This is a feature I'd also like to see

[perlindgren]

In the original RTFM, async (which is now schedule in the re-implementation) returns a handle to the message, allowing to abort the message. (One could also send that handle to some other task). Trying to abort an already released message was not supported.
/Per

[AfoHT]

I think with the current RTIC v1.0 this should be possible.

If not, please reopen.