Add back-off to mirroring script

[sethfitz]

Adding backoff will allow more jobs to run and not need to be retried when they fail.

Sample back-off in shell: https://gist.github.com/noahpryor/606c73f2208574356ec120b758147f3c

Mirroring script: https://github.com/mojodna/marblecutter-tilezen/blob/master/sentinel-2/mirror.sh