Clean up destination code and add some functionality

• Split the destinations into separate files
• New DestinationTempFile for writing tempfiles into the destination which eventually get added
• All of the close() methods are now streaming
• Implemented ChecksumDecorator, so we get a SHA1SUMS file
• Implemented FetchFile decorator, so we get a rudimentary fetch-all.sh script
• Configured Docker and delivery for local development to have a separate volume mount for serving files