## Problem
In https://github.com/neondatabase/neon/pull/8621, validation of keys
during ingest was removed because the places where we actually store
keys are now past the point where we have already converted them to
CompactKey (i128) representation.
## Summary of changes
Reinstate validation at an earlier stage in ingest. This doesn't cover
literally every place we write a key, but it covers most cases where
we're trusting postgres to give us a valid key (i.e. one that doesn't
try and use a custom spacenode).
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