Handle potential multiple "processing" range deletions on step-up

When a range deletion task starts, the relative document is marked as processing so that on step-up we can schedule the same range deletion before all the others.

Changing this if statament with a while would not impact the current semantics but would "protect" from potential bugs that may be introduced in the future, allowing resuming all "processing" deletions (or better, schedule them before all the others).

Even if at moment we don't expect more than one range deletion to be in such state, it would be wise to account for it anyway.