Add Type_handler::Item_sum_{sum|avg|variance}_fix_length_and_dec()

The following methods:

Item_sum_sum::fix_length_and_dec()

Item_sum_avg::fix_length_and_dec()

Item_sum_variance::fix_length_and_dec()

use methods cmp_type() and result_type() of their arguments.
The is not friendly to pluggable data types.
Pluggable data types should be able to define their own fix_length_and_dec() logic.

Under term of this task we'll split implementations of these methods into new methods in Type_handler:

virtual bool Item_sum_sum_fix_length_and_dec(Item_sum_sum *) const;

virtual bool Item_sum_avg_fix_length_and_dec(Item_sum_avg *) const;

virtual bool Item_sum_variance_fix_length_and_dec(Item_sum_variance *) const;

We'll also make the server return an error when a GOMETRY type expression appears as an argument of the affected operations.