In commit d74d9596 (MDEV-18543)
there was an error that would cause the hidden metadata record
to be deleted, and therefore cause the table to appear corrupted
when it is reloaded into the data dictionary cache.

PageConverter::update_records(): Do not delete the metadata record,
but do validate it.

RecIterator::open(): Make the API more similar to 10.6, to simplify
merges.