[VFS] configure cache size

[tobiasKaminsky]

Allow user to adjust cache size

• [ ] default, as now: unlimited
• [ ] 0b is possible (like on demand, which means that after sync to server local file is immediately deleted)

[PhilippSchlesinger]

suggestions:

• [ ] X% of storage space on device
• [ ] leave X% storage space on device
• [ ] X GB (fixed maximum cache size)

[tomdereub]

It would be great also to handle differently folders that have been manually set to "always keep offline". My proposal is :

• when the quota is reached, nextcloud frees up cache starting by the files that have been accessed for the longest time.
• do not free up the files and folders that have been manually set to "always keep offline". Maybe this files must not be counted in the cache quota ? Or are counted, but nextcloud starts to free up "automatic" cache before freeing up space in manually synced folders ?