* arangodb: bind list prefix, start file name and collection into the AQL query
Concatenating them into the query text let a caller-supplied prefix or
start name close the string literal and append arbitrary AQL, which runs
with the filer's ArangoDB credentials against any collection.
* arangodb: bind the folder path and collection into the recursive delete query
A trailing-slash S3 key reaches DeleteFolderChildren through the
directory-marker cleanup, so quotes in the path could turn the filter
into a match-everything REMOVE over the whole bucket collection.
* arangodb: match the real directory prefix in the recursive delete
The prefix was built by re-joining the path segments with commas, so it
never matched a stored directory and the subtree sweep did nothing.