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## Problem - After a shard split of a large existing tenant, child tenants can end up with oversized historic layers indefinitely, if those layers are prevented from being GC'd by branchpoints. This PR follows https://github.com/neondatabase/neon/pull/7531, and adds rewriting of layers that contain a mixture of needed & un-needed contents, in addition to dropping un-needed layers. Closes: https://github.com/neondatabase/neon/issues/7504 ## Summary of changes - Add methods to ImageLayer for reading back existing layers - Extend `compact_shard_ancestors` to rewrite layer files that contain a mixture of keys that we want and keys we do not, if unwanted keys are the majority of those in the file. - Amend initialization code to handle multiple layers with the same LayerName properly - Get rid of of renaming bad layer files to `.old` since that's now expected on restarts during rewrites.