Item_param::set_value() did not set Item::collation and
Item_param::str_value_ptr.str_charset properly. So both
metadata and data for OUT parameters were sent in a wrong
way to the client.

This patch removes the old implementation of Item_param::set_value()
and rewrites it using Type_handler::Item_param_set_from_value(),
so now setting IN and OUT parameters share the a lot of code.

1. Item_param::set_str() now:
  - accepts two additional parameters fromcs, tocs
  - sets str_value_ptr, to make sure it's always in sync with str_value,
    even without Item_param::convert_str_value()
  - does collation.set(tocs, DERIVATION_COERCIBLE),
    to make sure that DTCollation is valid even without
    Item_param::convert_str_value()

2. Item_param::set_value(), which is used to set OUT parameters,
   now reuses Type_handler::Item_param_set_from_value().

3. Cleanup: moving Item_param::str_value_ptr to private,
   as it's not needed outside.

4. Cleanup: adding a new virtual method
   Settable_routine_parameter::get_item_param()
   and using it a few new DBUG_ASSERTs, where
   Item_param cannot appear.

After this change:
1. Assigning of IN parameters works as before:
a. Item_param::set_str() is called and sets the value as a binary string
b. The original value is sent to the query used for binary/general logging
c. Item_param::convert_str_value() converts the value from the client
   character set to the connection character set

2. Assigning of OUT parameters works in the new way:
a. Item_param::set_str() and sets the value
   using the source Item's collation, so both Item::collation
   and Item_param::str_value_ptr.str_charset are properly set.
b. Protocol_binary::send_out_parameters() sends the
   value to the client correctly:
   - Protocol::send_result_set_metadata() uses Item::collation.collation
     (which is now properly set), to detect if conversion is needed,
     and sends a correct collation ID.
   - Protocol::send_result_set_row() calls Type_handler::Item_send_str(),
     which uses Item_param::str_value_ptr.str_charset
     (which is now properly set) to actually perform the conversion.