Improve support for transferring large files when operating on small nodes connected to external stores

Unified search : when "Search connected services" is unchecked, all the search providers that implement IExternalProvider and declare themselves as external must be hidden (not only ignored)

File cache table excessively large (and does not shrink after data removal) / Nextcloud should defragment file cache table

When moving files to another location, the old versions should be moved as well

Persistent external files daemon to mitigate per-process external files overhead

Write correct modification times on external storage (files_external)