## Problem
When a tenant creates a new timeline that they will treat as their
'main' history,
it is awkward to permanently retain an 'old main' timeline as its
ancestor. Currently
this is necessary because it is forbidden to delete a timeline which has
descendents.
## Summary of changes
A new pageserver API is proposed to 'adopt' data from a parent timeline
into
one of its children, such that the link between ancestor and child can
be severed,
leaving the parent in a state where it may then be deleted.
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Co-authored-by: Joonas Koivunen <joonas@neon.tech>
