Feature Request: Limit amount of parallel up-/downloads

Especially for Nextcloud on a weaker hardware (such as Raspberry Pis) it would be nice if we could limit the amount of parallel uploads or downloads (not just the bandwidth).

### Expected behaviour
If the number of parallel uploads is set to `1`: Just one file is uploaded to the server at the same time. The upload of file #II starts after upload of file #I has finished.
If the number is set to `2`: A maximum of two files is uploaded simultaneously..
...

### Actual behaviour
We can only limit the bandwidth used, not the number of parallel uploads. On e.g. a Raspberry Pi this means we have to move files into the synchronised folder one by one to avoid a server hiccup sometimes.

### Steps to reproduce
Put many files into a synchronised folder at once. The client will immidiately start to upload many of them at once. If the server is not capable of handling that many requests at the same time it will at least heavily slow down.

### Client configuration
Client version: 2.5.1final

Operating system: Windows

OS language: German

### Server configuration

Operating system:

Web server: Raspbian

Nextcloud version: 14

Storage backend (external storage): External HDD