Use same, simpler method to make only one thread open the export db as
is used for the ContentIdentifier db.
And, always update the export db once before using.
Had to add two more API calls to override export APIs that are not safe
for use in combination with import.
It's unfortunate that removeExportDirectory is documented to be allowed
to remove non-empty directories. I'm not entirely sure why it's that
way, my best guess is it was intended to make it easy to implement with
just rm -rf.
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.
