For now, it's only allowed when exporttree=yes is also set.
That simplified the implementation, but could later be changed if
there's a remote that makes sense to be an import but not an export.
However, it may work just as well to make a remote be readonly to
prevent export to it while still allowing import.
Made some api changes.
listImportableContents needs to provide the size
of the data, so the downloader can check disk free space.
retrieveExportWithContentIdentifier is passed the filepath to write to
Use temporary "CID" key during download of a ContentIdentifier from a
remote, so withTmp can be used and then move the content to the real key
once it's known.
