MDEV-33512 Corrupted table after IMPORT TABLESPACE and restart

In commit d74d95961a (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.

mysql-test/suite/innodb/r/instant_alter_import.result

@@ -61,6 +61,7 @@ alter table t1 discard tablespace;
 flush tables t2 for export;
 unlock tables;
 alter table t1 import tablespace;
+# restart
 select * from t1;
 z
 42

mysql-test/suite/innodb/t/instant_alter_import.test

@@ -79,6 +79,7 @@ flush tables t2 for export;
 unlock tables;
 
 alter table t1 import tablespace;
+--source include/restart_mysqld.inc
 select * from t1;
 
 drop table t2;