PROJQUAY-381 - failed mirror tag cleanup

[thomasmckay]

Issue: https://issues.redhat.com/browse/PROJQUAY-381

Changelog:
Fixed: Mirror of repos with a large number of tags may hang due to buffering skopeo output.
Fixed: When a mirror job is pre-empted, original tags may be deleted. Mirror jobs that are killed are less likely now to remove existing tags.

Docs:

Testing:

• Sync a repo with a lot of tags with debug logging enabled (eg. https://quay.io/repository/bitnami/sealed-secrets-controller ).
• Confirm job succeeds.
• Sync same repo again, cancelling after seeing the first skopeo inspect in the logs.
• Confirm job cancelled and any tags that were being synced are reverted in the tag history.

Details:
A hung mirror job that was either pre-empted or killed would not always rollback the changes. These changes fix the hanging but also move the deletion of old tags to after all images are synced.

[kurtismullins]

I have left suggestions that I believe should be taken into consideration. I won't block the PR on them, though. If you decide to change things and want another approval, just ping me.

Otherwise, it LGTM

[kurtismullins]

LGTM