Currently many items define its result_type() and then calculate field_type() and cmp_type() from result_type().

We'll change this to have items define its field_type(), while result_type() and cmp_type() will be calculated from field_type().

The change will consists of the following parts:
1. Changes in simple cases:

• Remove the default implementation of Item::cmp_type() and change the simple items (with non-hybrid types) to calculate cmp_type() from field_type():

  Item_result cmp_type() const { return type_handler()->cmp_type(); }

• Change the default implementation of Item::result_type() to detect the value by field_type(), instead of returning REAL_RESULT.
• Remove the default implementation of Item::field_type() and Implement field_type() for all Items that currently do not have it.
  The top level Item::field_type() will be pure virtual.

  Item_result result_type() const { return type_handler()->result_type(); }

• Rename the top-level Item::field_type() into a non-virtual Item::field_type_by_result_type(). It's still needed for Item_sum_sum, and will be removed in a later step.

2. Changes in Item_sum_sum:

• Remove Item_sum_sum::hybrid_type
• Derive Item_sum_sum from Type_handler_hybrid_field_type instead.
• Remove Item::field_type_by_result_type(), as it's not needed any more.

3. Changes in Item_splocal

• Remove Item_splocal::m_field_type and Item_splocale::m_result_type.
• Derive Item_splocal from Type_handler_hybrid_field_type instead.

4. Changes in Item_temporal_func

• Remove Item_temporal_hybrid_func::cached_field_type
• Derive Item_temporal_hybrid_func from Type_handler_hybrid_field_type

5. Changes in Item_param

• Remove Item_param::param_type and Item_param::item_result_type
• Derive Item_param from Type_handler_hybrid_field_type

6. Changes in Item_type_holder

• Remove Item_type_holder::fld_type
• Add type handlers for all missing possible Field::real_type() values. Note: we have had handlers for all field_type() values only so far.
• Add Type_handler_hybrid_real_field_type, derived from Type_handler_hybrid_field_type
• Derive Item_type_holder from Type_handler_hybrid_real_field_type

7. Canges in Item_cache

• Remove Item_cache::cached_field_type
• Derive Item_cache from Type_handler_hybrid_field_type

8. Changes in Item_copy

• Remove Item_copy::cached_field_type and Item_copy::cached_result_type
• Derive Item_copy from Type_handler_hybrid_field_type