Register a DUAL table by default

[smola]

The DUAL table present in MySQL is often used by 3rd party tools, connection pools, etc.

Registering it by default should help us to improve compatibility our of the box. It could be implemented easily with a mem.Table.

[ajnavarro]

This table is a little bit special, it shouldn't appear when you execute show tables, so maybe we need to add a new method to Table node to know if the table is hidden or not.

[smola]

@ajnavarro Maybe we don't care if it appears in SHOW TABLES initially?