3.0 table isUnique

This adds a unique value validator for the Table object. I had trouble maintaining the same API we had in 2.x, due to the heavy modifications done to the validator, but it actually helped me implementing the most common cases I've had regarding uniqueness validation:

* Checks for uniqueness as long as the record is not the same during an update
* Scopes the uniqueness to another field in the table, which is useful for multi-tenancy

Closes https://github.com/cakephp/cakephp/issues/3449