Bug: stats.lua cannot handle deep nesting

[sebbASF]

stats.lua tries to calculate message threading.

However if too many messages reference each other, the code crashes with an error:

site/api/stats.lua:755: Cannot serialise, excessive nesting (501)

This prevents the list data from being displayed.

The code should fail gracefully if the nesting depth is too great.

[sebbASF]

It appears that Jenkins links multiple runs of the same job together; each run references the previous one.