Consider disk usage of collDir instead of default /crawls

[benoit74]

Fix #585

Changes:

• compute disk usage based on crawler collDir property instead of always computing it on /crawls directory

[benoit74]

@tw4l @ikreymer this is ready for review from my PoV, I don't get why there are three failing tests which seems very unrelated from my experience (but still very limited compared to yours)

[tw4l]

Thanks for this @benoit74! Looks like the tests are okay now, maybe that was just a fluke/bad run. Will plan to review this week.

[ikreymer]

thanks, yep, i think that makes sense! I think we set it to /crawls in case multiple crawls are running using same volume locally, but that should still work fine and allows for more granular checking