Clean up local tarball and backup dir after successful S3 upload. 

This patch deletes the backup dir and tarball after successful S3 upload.

I attempted to clean up unconditionally, but keep running into a bizarre EPIPE error related to either asynchronous logging to stdout after process ends or related to the exec calling `rm -rf`. At least cleaning up on success is closer to the right behavior than current behavior, which will quickly fill up a filesystem for very large databases.