Implement min_spacing for actions

[erlingrj]

min_spacing is a optional argument to an action restricting the minimum time delta
between two events scheduled on the action.

It was easy to support when we required actions scheduled monotonically. But now that we will support arbitrary schedulings (#71) we need a complete implementation.

A proposed algorithm looks like this:
When lf_schedule is called with a tag t, if there is a min_spacing. We must search through the event queue and guarantee that there are no other events from the same action scheduled in the time-window" [t-min_spacing, t+min_spacing]. If so, drop the event.

In LF there are actually several policies for handling such an "illegal" schedule:

• defer
• drop
• replace

However, is this well-defined if you try to schedule an event "between" two existing events on the event queue? Where do you defer it to? Which of the two events do you replace?

I think for now we will stick with the drop policy

[edwardalee]

If you look at the specification of min_spacing here: https://www.lf-lang.org/docs/next/writing-reactors/actions#triggering-time-for-actions, then I think the implementation is very simple. When you give a min_spacing, then monotonic scheduling becomes required. I think the current implementation in reactor-c takes advantage of this.

[erlingrj]

OK, I see, in this case it is very simple, I will look at it