Syntax for CONVERT TABLE

ALTER TABLE tbl_name CONVERT TABLE tbl_name TO PARTITION partition_name partition_spec

Examples:

    ALTER TABLE t1 CONVERT TABLE tp2 TO PARTITION p2 VALUES LESS THAN MAX_VALUE();

New ALTER_PARTITION_CONVERT_IN command for
fast_alter_partition_table() is done in alter_partition_convert_in()
function which basically does ha_rename_table().

Table structure and data check is basically the same as in EXCHANGE
PARTITION command. And these are done by
compare_table_with_partition() and check_table_data().

Atomic DDL is done by the scheme from MDEV-22166 (see the
corresponding commit message). The only differnce is that it also has
to drop source table frm and that is done by WFRM_DROP_CONVERTED_FROM.

Initial patch was done by Dmitry Shulga <dmitry.shulga@mariadb.com>