Implement "drop" and "replace" policy for actions

[erlingrj]

#170 added support for min spacing of events scheduled on actions. It uses the default "defer" policy to handle violations. LF also defines "drop" and "replace" which we should implement. "drop" is very simple, simply drop the event and return. "replace" requires us to search through the event queue to find the last (?) pending event of an action and the replace the payload of this event (also we must remember to free the old payload). This will require some addition to the EventQueue API, but nothing too bad.