Because minetest.after() is not bound to a position, and because it is not
saved, it may trigger after the node to which it applies was unloaded, or
not at all (if the session ends). If that happens, The 'young' flag
will not be cleared, and will have become persistent (i.e. without a timer
which will eventually clear it).
This patch removes the 'young' flag, and replaces it with a timestamp, removing
the need for a timer. Any lingering 'young' flags that have become persistent
are automatically removed over time (i.e.: they are simply ignored...)
