(PUP-1044) Ensure file bucket streams are closed at all times after use

This makes the stream() method on file bucket file take a block
to ensure that the processing of the stream always ends up closing it.
Merge pull request #2999 from hlindberg/PUP-1044_ensure-closed-filebucket-file

(PUP-1044) Ensure file bucket streams are closed at all times after use