Allow schedule_after(0, ...) to work

[shawntabrizi]

Right now, if a user puts schedule_after(0, ...), the call will fail because there is a check that the scheduled block must be greater than the current block:

if when <= now {
	return Err(Error::<T>::TargetBlockNumberInPast.into())
}

This PR simply adds one additional block to any schedule_after call. So schedule_after(0, ...) will be the block after the current block, schedule_after(1, ...) will be the block after that, etc... So after this PR, all schedule_after will happen one block later. This allows schedule_after to always pass this check with any after input.

This is just a quality of life improvement which should capture the intention of the caller.

cc @xlc

[bkchr]

@shawntabrizi you need to merge master

[shawntabrizi]

bot merge

[ghost]

Trying merge.