This patch:
- Introduces a new virtuial method Type_handler::set_comparator_func
  and moves pieces of the code from the switch in
  Arg_comparator::set_compare_func into the corresponding
  Type_handler_xxx::set_comparator_func.
- Adds Type_handler::get_handler_by_cmp_type()
- Moves Type_handler_hybrid_field_type::get_handler_by_result_type() to
  a static method Type_handler::get_handler_by_result_type(),
  for symmetry with similar methods:
  * Type_handler::get_handler_by_field_type()
  * Type_handler::get_handler_by_real_type()
  * Type_handler::get_handler_by_cmp_type()
- Introduces Type_handler_row, to unify the code for the scalar
  data types and the ROW data type (currently for comparison purposes only).
- Adds public type_handler_row, as it's now needed in item_row.h
- Makes type_handler_null public, as it's now needed in item_cmpfunc.h
  Note, other type_handler_xxx will become public as well later.
- Removes the global variable Arg_comparator::comparator_matrix,
  as it's not needed any more.