Allow instant changes of columns in ROW_FORMAT=REDUNDANT
from NOT NULL to NULL.

Later, this may be implemented for ROW_FORMAT=COMPACT or DYNAMIC,
but in that case any indexes on the table must be rebuilt.

dict_table_t::prepare_instant(): Add some debug assertions,
and relax a debug assertion so that the number of fields is
allowed not to change.

dict_index_t::instant_add_field(): Relax a debug assertion,
allowing a column to change from NOT NULL to NULL.

dict_table_t::instant_column(): Add debug assertions.

instant_alter_column_possible(): Allow ALTER_COLUMN_NULLABLE
when applicable.

innodb_insert_sys_columns(): Add the parameter bool update=false
to run UPDATE instead of INSERT.

innobase_instant_add_col(): Remove; let the only caller invoke
innodb_insert_sys_columns() directly.

innobase_instant_try(): Update the SYS_COLUMNS record if the
column is changed. Only convert the table to the instant ALTER TABLE
format if necessary. For ALTER_COLUMN_NULLABLE in ROW_FORMAT=REDUNDANT,
there is no data format change.