Missing insert source aliases parsing

Insert statements should be able to alias sources for use in on duplicate expressions:
```sql
INSERT INTO t1 (a,b,c) VALUES (1,2,3),(4,5,6) AS new(m,n,p)
ON DUPLICATE KEY UPDATE c = m+n;
```
We should support the optional `[AS row_alias[(col_alias [, col_alias] ...)]]` in between the values list and on duplicate clause.

The workaround right now uses `VALUES(field)` rather than disambiguating, but this is discouraged in mysql.