Tested with nodebreaker, fire.
If called from lua, minetest.remove_node() calls on_destruct() callbacks
before the map is actually updated. This means that we can't look at the
map data to determine if we're done cleaning up adjacent nodes, and we
have to stop recursing some other way.
There's no data we can pass around through functions that would survive
scope to a secondary on_destruct() callback, so we have to maintain
local state somewhere in the mod namespace.
In this case, we keep a bitflag. The bitflag is set to "true" by
default. On the first half removal, the flag is flipped and afterwards
we remove the other half node. When the on_destruct for the other half
is running, it's value is false and we flip it back to true without
removing the other half node.
This thus prevents recursing.
To facilitate easier finding of the bed partner, we tell our on_destruct
whether we're a top or bottom half node through a passed flag.
Now that the top is diggable, we just need to assure that it drops a
bottom bed part.
