Split resolve_const_item() into virtual methods in Type_handler

resolve_const_item() is not friendly to pluggable data types, as it has:

• A switch on the result of item_cmp_type()
• A call for field_type_for_temporal_comparison()

This is hard-coded for built-in data types only and won't work properly for new data types.

We'll do the following:

• Introduce a new method in Type_handler:

  virtual Item *make_const_item_for_comparison(THD *thd,

  Item *src,

  Item *comparand) const= 0;

• Split the code in resolve_const_item() into virtual implementations of the proper type handler implementations
• Replace item_cmp_type() to Type_handler_hybrid_field_type::aggregate_for_comparison()
• Remove Item::field_type_for_temporal_comparison(), as it won't be used any more in the server code.