Here's my version of the mod. I started out just trying to fix the intermittent crashes, mostly when a player leaves. I ended up doing major refactoring.
There were several places where the original logic (0.6 release) was broken. There were workarounds in place to try to correct it (temporarily adding cobblestone block, etc). These are no longer necessary and have been removed.
This version is much simpler and faster.
I've tested on minetest 0.4.10 without luajit and minetest 0.4.13 with luajit. I can't get walking light to crash the server any more.
https://github.com/myoung008/walking_light