Implement cleanup for expired recoveryfiles

[jimboid]

Recovery files tend to build up rapidly over time, currently the user has to periodically clean these out by themselves. A better solution would be either of:

1. Only keep a small number (say the last week)
2. Provide a command-line flag such as --recovery-clean and just wipe out anything in the directory with it.
3. Rearrange how the file is handled within the code. Use the global jobfile parameter simply for initial creation. Then store a handle to the file under each job (no other place to put it). This way can make use of the filename throughout Longbow, no need to create new recovery file on recovery or reconnections and can add code to the cleanup function to kill the recovery file once all jobs are done.