Add option to `files_external:option` and `files_external:config` to remove a field

files_external: limited number of connections

[files_external] Changing an External Storage mount backend or path silently destroys all metadata (previews, Albums, Recognize, Maps, shares)

files_external amazons3: cross-mount MOVE should use server-side MultipartCopy when peer is same S3 endpoint

Write correct modification times on external storage (files_external)