For all of the small number of uses timeline ancestor detach (#6994) has seen, there is a need to download layers in the restart following completion of the prepare phase after reset_tenant. The on-demand downloads happen for logical size calculation or compaction.

We could simply hard link all Layer instances which happen to be present already. Layer::keep_resident can be used to test for the residency, and keeping the layer resident while hard-linking happens. There are already TODO comments, and the way we fsync might need to change as well.

This is a performance optimization, but it is easy to do.