Support the definition of null change to not null using alter table

[zimulala]

Refer to https://dev.mysql.com/doc/refman/5.7/en/alter-table.html.

[winkyao]

If we have no global lock on the table, it is not a trivial work.

[ciscoxll]

Relate to PR #6217 .

[morgo]

I have verified two test cases, behavior is the same as MySQL:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a int not null primary key auto_increment, b INT NULL);
INSERT INTO t1 (b) VALUES (1);
ALTER TABLE t1 CHANGE b b INT NOT NULL;

And an error:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a int not null primary key auto_increment, b INT NULL);
INSERT INTO t1 (b) VALUES (NULL);
ALTER TABLE t1 CHANGE b b INT NOT NULL;

# Result:
ERROR 1138 (22004): Invalid use of NULL value

I am going to close this task now.