page_direction_reset(): Reset PAGE_DIRECTION, PAGE_N_DIRECTION.

page_direction_increment(): Increment PAGE_N_DIRECTION
and set PAGE_DIRECTION.

We must know the size of each entry in the PAGE_FREE list.
After rolling back an instant ADD COLUMN operation, if the
'default row' is put into the PAGE_FREE list, we would be
unable to determine its size (if it is in ROW_FORMAT=COMPACT
or ROW_FORMAT=DYNAMIC) because it would contain more fields
than the rolled-back definition of the clustered index.

For instant ADD COLUMN, the dictionary cache and the persistent
data structures will be updated in
ha_innobase::commit_inplace_alter_table(). The dict_table_t::cols[]
will be updated before the transaction is committed or rolled back.
If the transaction is rolled back, the changes must be reverted.

dict_index_t::instant_add_field(): Instantly add clustered index fields
to an existing index.

dict_table_t::instant_add_column(): Instantly add columns
to an existing table definition.

dict_table_t::rollback_instant(): Roll back instant_add_column().

typo fixed

The adaptive hash index is built on record prefixes on leaf pages,
containing at most index->n_uniq fields. Because the info_bits are
not part of the hash value calculation, the adaptive hash index
cannot distinguish the 'default row' record from a genuine user record
(and it cannot distinguish whether a record is delete-marked).

rec_fold(): Move the function to the only module where it is used,
and assert that the record is on a leaf page and not the 'default row'.

btr_search_drop_page_hash_index(), btr_search_build_page_hash_index():
Skip the 'default row'

btr_cur_optimistic_delete_func(): Do not try to discard a lock on the 'default row' because must be there none

MDEV-13953 mariadb-10.2.9/storage/innobase/row/row0import.cc:3602: possible cut'n'paste error ?
MDEV-13955 mariadb-10.2.9/sql/log_event.cc:7671: possible wrong operator ?

Only page_create_low() and PageBulk::finish() are entitled to reset
PAGE_INSTANT. page_copy_rec_list_end_to_created_page() may be invoked
on the root page, where PAGE_INSTANT would be set.

update engine tests