Separating Value from Item

Make Value independent from Item, so Item can pass it and change (may be store also).

Why value

We're trying to solve the following goals by adding Value:

1. A step to make Items reentrant
2. Improve performance
3. Reduce the code size by removing some data type specific functions/methods
4. Pass data to plugins

The current value implementation

There is already struct st_value and a method in Item:

  bool save_in_value(THD *thd, struct st_value *value)

  {

    return type_handler()->Item_save_in_value(thd, this, value);

  }

Monty will write why the current implementation is bad for the intended goals.

For now it should be:

• Null flag
• reference on Type
• DATA