rec_get_offsets(): Use the 'leaf' parameter for non-debug purposes.
Initialize all fields for ROW_FORMAT=REDUNDANT records that precede
an instant ADD COLUMN.

rec_offs_make_valid(): Add the parameter 'leaf'.

rec_copy_prefix_to_dtuple(): Assert that the tuple is only built
on the core fields. Instant ADD COLUMN only applies to the
clustered index, and we should never build a search key that has
more than the PRIMARY KEY and possibly DB_TRX_ID,DB_ROLL_PTR.
All these columns are always present.

dict_index_build_data_tuple(): Remove assertions that would be
duplicated in rec_copy_prefix_to_dtuple().

rec_get_n_nullable(): Get the number of nullable columns.

rec_init_offsets(): Support ROW_FORMAT=REDUNDANT records that
were written before instant ADD COLUMN.

cmp_rec_rec_with_match(): Implement comparison of MIN_REC_FLAG records.