closure: Clear closure functions in idle handler

When closures are invalidated, they need to drop the reference to their
JS functions. However, it appears that closure invalidate notifications
can be sent out during garbage collection. Incremental GC does not allow
changing what you trace while GC is going on, so we have to keep tracing
the JS functions, and only drop them in an idle handler later on.

We still decrement the memory counter in the invalidate notification,
though, because if you are doing a memory counter check at the end of
your program, the idle handler may not have run yet.

https://bugzilla.gnome.org/show_bug.cgi?id=776966