## Problem
In preparation to https://github.com/neondatabase/neon/issues/9516. We
need to store rel size and directory data in the sparse keyspace, but it
does not support inheritance yet.
## Summary of changes
Add a new type of keyspace "sparse but inherited" into the system.
On the read path: we don't remove the key range when we descend into the
ancestor. The search will stop when (1) the full key range is covered by
image layers (which has already been implemented before), or (2) we
reach the end of the ancestor chain.
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Signed-off-by: Alex Chi Z <chi@neon.tech>
e9ed53b14f