nexus-writer: finer grained separation of files in local directory

[DanNixon]

Problem this feature will solve

The nexus-writer should be able to handle the case where files cannot be copied to the archive directory.
Currently, it cannot cope with the situation where a copy fails.

Description of solution

• Have two directories in the local directory: working and completed
• working should only ever contain one file: the one currently being written
• Any files in working on startup are assumed to be incomplete after a crash and are handled as they currently are
• When a file is completed, it is first moved from working to completed, after this it can be attempted to be copied to the archive
• Only after a successful copy should the file in completed be removed
• nexus-writer should try on startup and periodically to move any files still in completed to the archive, the interval should be configurable