Support drop and add primary key in one statement

[tangenta]

The following case is useful in reorganizing primary without the risk of duplicated primary key:

CREATE TABLE t1 (id int primary key, c1 int not null);
ALTER TABLE t1
  DROP PRIMARY KEY,
  ADD PRIMARY KEY (id, c1);

Expected:

Query OK, 0 rows affected (0.01 sec)

But got:

mysql> ALTER TABLE t1
    ->   DROP PRIMARY KEY,
    ->   ADD PRIMARY KEY (id, c1);
ERROR 8200 (HY000): Unsupported drop primary key when the table's pkIsHandle is true