This commit blocks concurrent calls to RunTaskInBackground which is
intended to fix the quirky behaviour where concurrent PUT calls to
api/publish/<prefix>/<distribution> would immedietly reuturn an error.
The solution proposed in this commit is not elegant and probaly has
unintended side-effects. The intention of this commit is to highlight
the area that actually needs to be addressed.
Ideally this patch is amended or dropped entierly in favor of a better
fixup.