Fix multiprocessing bugs, add unlink_worker
Fix multiprocessing bugs, add unlink_worker

This is still under some live development, for sure :-)

Among other minor (yet significant) fixes, I've added another
worker process to handle unlinking.  This verifies that the file
was transmitted properly (or at least with the right md5sum)
before unlinking it.

rtucker/backuppc-archive-s3,rtucker/backuppc-archive-s3