File stuck in processing state forever

## Describe the bug
Since yesterday, I have about ~10 files stucks in processing (visible in the web ui) in one of my OCIS space (140k files, ~70Gb).
At first, I tried the maintenance commands:
- `ocis storage-users uploads sessions` => there were a lot of (very old) sessions.
- `ocis storage-users uploads sessions --resume` => cleaned most (except 4)
- `ocis storage-users uploads sessions --clear` => to remove the last 4 ones
- `ocis storage-users upload sessions delete-stale-nodes` => nothing, files stuck in processing are still there.

What is even more strange is that files haven't been modified for a while:

## Steps to reproduce
Here is what I've done to reach the current state :
On a computer with files already synced, I removed an re-added the synchronization of my space in the owncloud desktop client.
I don't know why, but it generated a lot of "upload sessions" ! (despite the files being there and correct).
There were several errors during this phase, but at the end almost everything was there, except a few files which are now locked in processing state. I also noticed that file processing is quite slow (~4/second, with an i3-10100).

Note : I tried to re-setup the synchronization of my space because of [another bug](https://github.com/owncloud/client/issues/12231) in the desktop client.

## Expected behavior
- files shall never be stuck in processing forever
- file processing shall be fast
- there shall be a way to deal with those stuck files with maintenance commands

## Setup
- ocis 7.2.0
- client: ownCloud version 5.3.2.15463 (linux)