In the case when all unique key fields are nullable, there can be
several records with all NULL key fields in unique index. When
transaction is resumed after waiting on the record with all key fields
equal to NULL, and stored in persistent cursor record is deleted,
persistent cursor can be restored on the record with all key fields equal
to NULL, and such record is wrongly treated as the record with the same
unique key as the stored in persistent cursor record, what is wrong.

The fix is to check if all unique fields are null when persistent cursor
position is restored and not all comparing fields are equal, and if so,
don't treat the record as a record with the same unique key as in the
stored in persistent cursor record key.

dict_index_t::nulls_equal was removed, as it was initially developed for
never existed in MariaDB "intrinsic tables", and there is no code, which
would set it to "true".

Reviewed by Marko Mäkelä.