This momentarily breaks the iterator fault injection test, but I would
have had to squash too many commits to make it work right, so it
appeared to be the better option to disable the respective test for some
commits.

this is to make reading disk objects more efficient later on.

This patch triggered c93.vtc failing, so we add a bit of a hack to avoid
a problem which might need more attention later:

FCO_MAX_REGIONS is an issue for chunked encoding objects (with growing
size). We have not yet implemented the best strategy and just tried
to always allocate the largest possible seglist to not use up more
regions than necessary, but small memory configurations do not support
the maximum seglist (4 MB).