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...)
If moretrees is not present, nature_classic spawns blossoms and
eventually apples on default trees, as it always has.
If moretrees IS present, the code will only spawn apple blossoms on
moretrees' own apple tree leaves, and will only grow new apples on those
trees, not on default trees. In this case, the nature:blossom node is
considered deprecated and is aliased to default:leaves to avoid causing
undefined nodes in old maps.
