column prefix of an externally stored column.

row_upd_ext_fetch(): New function.

row_upd_index_replace_new_col_vals(),
row_upd_index_replace_new_col_vals_index_pos(): Fetch prefixes of
externally stored columns when they are needed for column prefix
indexes.  For memory allocation, add the parameter ext_heap.  Avoid
repeating the inner loop after finding a  matching upd_field->field_no.