RPM Support

It seems to happen because the mirror implementation from the pulpcore is used which is unaware of the RPM specific optimize option which does not add any content to a new incomplete repo version.

Possible solutions:

1. have custom implementation of DeclarativeVersion.create in the plugin
2. have plugin input in the pulpcore when deciding whether to add the content unassociation step or not
3. add existing content to the incomplete repo version, so it would be clear that content is there and there is just nothing to remove because it's the same.
4. anything else?

I looked over the optimize_sync code and I recommend moving it out of RpmFirstStage and into the task code called before DeclarativeVersion.create() is even called. This will cause the pipeline never to start. From an efficiency perspective since no DeclarativeContent is emitted down the pipeline there is no concurrency opportunity so handling that synchronously earlier in the code should be the same from an efficiency perspective.