ddl: support multi-table statements

[sougou]

rename and drop allow multiple tables in the DDL.
This change adds support for them.

• Introduced two new members to DDL: 'from' and 'to' table lists.
• NewName is deprecated.
• For single table DDLs, only Table is set.
• For multi-table DDLs, 'from' and 'to' tables are set as needed.
• The vttablet ddl plan had unused fields. They've been deleted.
• The permissions code has been updated to use the new table names.

Signed-off-by: Sugu Sougoumarane ssougou@gmail.com

[sougou]

@slanning you may also review this.

[slanning]

I didn't manage to break it yet, so looks good to me.

[sougou]

These JSONs are not for ast.DDL. They're for planbuilder.DDLPlan: https://github.com/vitessio/vitess/blob/master/go/vt/vttablet/tabletserver/planbuilder/ddl.go#L24.

Basically, DDLPlan used to have TableName and NewTable, but they were unused. So, I removed them. That leaves it with Action as the only member. In a previous life, vttablet used to conditionally load/unload schema info for those tables. But now, it just passes the DDL through. So, it doesn't need to know the table names any more.

In a way, even Action is mostly ignored, but there's one flow where an empty action is treated as an error: https://github.com/vitessio/vitess/blob/master/go/vt/vttablet/tabletserver/query_executor.go#L394.

As to why it accepts "XYZ": "b": it's because JSON is permissive. It just ignores unmatched fields.

No changes needed