Avoid job parallelism when uploading to an encrypted folder

Avoid job parallelism when uploading to an encrypted folder

With the current design of the file upload this necessarily pushed to a
lock starvation on the folder. Indeed you could end up with N jobs
asking for the lock at the same time. So just avoid parallelizing for
now even though it will be slow.

We could try to optimize but that'd require some serious changes to the
sync logic on the jobs... let's stabilize first and optimize later.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>