nodestack: implement expiration sloppiness parameter

Now, a previously-explored, repeated future value will only generate a
new future-value/expiration pair if the expiration is higher than the
previous expiration + sloppiness. This prevents executions where we
continue to step up the future value expiration by a small increment,
creating the same execution.

This also rewrites the add_future_value() logic just a bit so that it is
a little bit more clear.