Allow validating multiple Data Sets at once

[maxpatiiuk]

Currently, the second DS would be stack in queue until the first one is done.
Same applies for upload and rollback.

[maxpatiiuk]

There is probably much more to this, but the first step might be to modify --concurrency=1 here:

specify7/docker-compose.yml

Line 83 in 68200c2

command: ve/bin/celery -A specifyweb worker -l INFO --concurrency=1 -Q specify

(and same in production compositions)

and then to adapt the back-end for that change