We must use the original page format for inserting metadata records.
This is because in case the instant ALTER TABLE operation needs to
be rolled back (due to crash recovery), all pages must remain in
the original format.

btr_cur_optimistic_insert(): Avoid reorganize for inserting the
metadata record.

btr_cur_optimistic_update(): Initialize rec,offsets correctly
after possible page format conversion.

innobase_instant_try(): Do not store NULL values for those
metadata record columns that were originally declared NOT NULL.

dict_index_t::get_n_nullable(): Refer to dict_col_t::was_not_null()
in order to keep the original format intact.

lock_move_reorganize_page(): Allow the two pages to be in
different formats.

rec_convert_dtuple_to_rec_comp(), rec_convert_dtuple_to_rec_comp():
Also observe the DATA_WAS_NOT_NULL flag.