Ratelimit RSS-triggered GCs

When loading a lot of data in memory (for example in the shell
opening the overview, which loads all the desktop files and icons)
the RSS can increase a lot, so we would trigger GCs continously
without any hope of freeing memory, so ratelimit full GCs to at
most one every 5 frames.

https://bugzilla.gnome.org/show_bug.cgi?id=728048