Extend optimize feature to mirror=True sync policy

[quba42]

optimize mode currently does not work in combination with mirror=True sync policy, so we do not use them together: #630

The reason is that since optimize mode does not add anything, mirror mode then goes ahead and deletes everything from the repo "since it is no longer in the upstream repo".

Our first attempt at using optimize in combination with mirror ended up in a dead end.

The new plan is to replace the pulpcore mirror option with a dedicated AptMirror stage, that is capable of mirroring individual distributions or components instead of just the entire new repository version.

[quba42]

This issue should revert 881f198

[quba42]

We consider the full featured partial optimize mode to be blocked until we have subrepositories in pulp_deb. In the meantime we want to start with the most important special case, which is relatively easy to fix now: #1027