Split Item_func_round::fix_length_and_dec to virtual methods in Type_handler

Future new data types might want the function ROUND:

• to do something different than INT,DECIMAL,DOUBLE rounding
• or even return an error (if this operation is not applicable)

We'll split the following switch in Item_func_round::fix_length_and_dec

switch (args[0]->result_type())

to virtial methods in Type_handler.

Also, we'll disallow ROUND for the GEOMETRY type, as it's useless. So GEOMETRY will be an example of a type whose ROUND operation is not applicable.