Split Item::tmp_table_field_from_field_type() into virtual methods in Type_handler

Currently Item::tmp_table_field_from_field_type() has this code:

  switch (field_type()) {

  case MYSQL_TYPE_DECIMAL:

  case MYSQL_TYPE_NEWDECIMAL:

  ...

  case MYSQL_TYPE_TINY:

  ...

  case MYSQL_TYPE_SHORT:

  ...

This is not friendly to pluggable data types.

We'll introduce a new method in Type_handler:

virtual Field *make_table_field(const LEX_CSTRING *name,

                                const Record_addr &addr,

                                const Type_all_attributes &attr,

                                TABLE *table) const= 0;

and move all switch pieces into implementations of make_table_field of the corresponding Type_handler_xxx.

Note, the new method for now will be used to create only fields for temporary tables, but later we'll reuse it for make_field() purposes in field.cc. Therefore, the API for the new method contains a new class Record_addr which describes pointer and null-pointer of this field in the record.

class Record_addr

{

public:

  uchar *ptr;      // Position to field in record

  /**

     Byte where the @c NULL bit is stored inside a record. If this Field is a

     @c NOT @c NULL field, this member is @c NULL.

  */

  uchar *null_ptr;

  uchar null_bit;  // Bit used to test null bit

  Record_addr(uchar *ptr_arg,

              uchar *null_ptr_arg,

              uchar null_bit_arg)

   :ptr(ptr_arg),

    null_ptr(null_ptr_arg),

    null_bit(null_bit_arg)

  { }

  Record_addr(bool maybe_null)

   :ptr(NULL),

    null_ptr(maybe_null ? (uchar*) "" : 0),

    null_bit(0)

  { }

};

Additionally we'll remove the fixed_length and set_blob_packlength parameters from Item::tmp_table_field_from_field_type.
Reproducing these parameters in the new method make_table_field would look too dirty.

Removing fixed_length is easy: it's not really used, the code checking fixed_length in Item::tmp_table_field_from_field_type is dead code.

Removing set_blob_packlength is less trivial. This parameter was needed to choose a proper blob type handler (type_handler_blob, type_handler_tiny_blob, type_handler_medium_blob, type_handler_long_blob) depending on max_length. So instead of choosing the exact data type during tmp_table_field_from_type(), we'll fix all affected Item_xxx::type_handler() to return a correct type handler, which is in sync with max_length. Some Item_xxx will need to fix type_handler() or field_type(), other will need to fix fix_length_and_dec().