MDEV-12833 Split Column_definition::create_length_to_internal_length() to virtual methods in Type_handler

MDEV-12826 Add Type_handler::val_int_signed_typecast() and Type_handler::val_int_unsigned_typecast()

- Adding new virtual methods in Type_handler:
  * Column_definition_prepare_stage1()
  * Column_definition_prepare_stage2()
  * calc_pack_length()

- Using new methods to remove type specific code in:
  * Global function calc_pack_length()
  * Column_definition::prepare_create_field()
  * The loop body mysql_prepare_create_table()
  * Column_definition::sp_prepare_create_field()

Introducing a new class Type_holder (used internally in sql_union.cc),
to reuse exactly the same data type attribute aggregation Type_handler API
for hybrid functions and UNION.

This fixes a number of bugs in UNION:

- MDEV-9495 Wrong field type for a UNION of a signed and an unsigned INT expression
- MDEV-9497 UNION and COALESCE produce different field types for DECIMAL+INT
- MDEV-12594 UNION between fixed length double columns does not always preserve scale
- MDEV-12595 UNION converts INT to BIGINT
- MDEV-12599 UNION is not symmetric when mixing INT and CHAR

Details:

- sql_union.cc: Reusing attribute aggregation for UNION.
  Adding new methods:
  * st_select_lex_unit::join_union_type_handlers()
  * st_select_lex_unit::join_union_type_attributes()
  * st_select_lex_unit::join_union_item_types()
  Removing the old join_types()-based code.

- Changing Type_handler::Item_hybrid_func_fix_attributes()
  to accept "name", Type_handler_hybrid_field_type, Type_all_attributes
  as three separate parameters instead of a single Item_hybrid_func parameter,
  to make it possible to pass both Item_hybrid_func and Type_holder.

- Moving the former special GEOMETRY and ENUM/SET attribute aggregation code
  from Item_type_holder::join_types() to
  * Type_handler_typelib::Item_hybrid_func_fix_attributes().
  * Type_handler_geometry::Item_hybrid_func_fix_attrubutes().
  This makes GEOMETRY/ENUM/SET symmetric with all other data types
  (from the UNION point of view).
  Removing Item_type_holder::join_types() and Item_type_holder::get_full_info().

- Adding new methods into Type_all_attributes:
  * Type_all_attributes::set_geometry_type() and
    Item_hybrid_func::set_geometry_type().
  * Adding Type_all_attributes::get_typelib().
  * Adding Type_all_attributes::set_typelib().

- Adding Type_handler_typelib as a common parent for
  Type_handler_enum and Type_handler_set, to avoid code duplication: they have
  already had two common methods, and we're adding one more shared method.

- Adding Type_all_attributes::set_maybe_null(), as some type handlers
  may want to set maybe_null (e.g. Type_handler_geometry) during data type
  attribute aggregation.

- Changing Type_geometry_attributes() to accept Type_handler
  and Type_all_attributes as two separate parameters, instead
  of a single Item parameter, to make it possible to pass Type_holder.

- Adding Item_args::add_argument().

- Moving Item_args::alloc_arguments() from "protected" to "public".

- Moving Item_type_holder::Item_type_holder() from item.cc to item.h, as
  now it's very simple.
  Btw, this constructor should probably be eventually removed.
  It's now used only in sql_show.cc, which could be modified to use
  Item_return_decimal (for symmetry with Item_return_xxx created for all
  other data types). Or, another option: remove all Item_return_xxx and
  use Item_type_holder for all data types instead.

- storage/tokudb/mysql-test/tokudb/r/type_float.result
  Recording new results (MDEV-12594).

- mysql-test/r/cte_recursive.result
  Recording new results (MDEV-9497)

- mysql-test/r/subselect*.result
  Recording new results (MDEV-12595)

- mysql-test/r/metadata.result
  Recording new results (MDEV-9495)

- mysql-test/r/temp_table.result
  Recording new results (MDEV-12594)

- mysql-test/r/type_float.result
  Recording new results (MDEV-12594)

MDEV-12798 Item_param does not preserve exact field type in EXECUTE IMMEDIATE 'CREATE TABLE AS SELECT ?' USING POINT(1,1)

This push just fixes tests, nothing wrong in server

"mtr --ps func_str" failed:
pad_str was erroneously appended with a new space on every PS execution.

Adding pad_str.length(0) into fix_length_and_dec() to avoid this.

This patch for MDEV-12770 is also fixing:

MDEV-12769 Arithmetic operators with temporal types create excessive column types

- SETVAL(sequence_name, next_value, is_used, round)
- ALTER SEQUENCE, including RESTART WITH

Other things:
- Added handler::extra() option HA_EXTRA_PREPARE_FOR_ALTER_TABLE to signal
  ha_sequence() that it should allow write_row statments.
- ALTER ONLINE TABLE now works with SEQUENCE:s

Other things
- Ensure that ut_d() is set to EXPR if ut_ad() is DEBUG_ASSERT()
  If not, we will get a crash in purge_sys_t::~purge_sys_t() as
  this ut_ad() code expect's that the ut_d() codes has been executed

Adding "const" qualifier to Item::cols(), and to the "Item *cmp" parameter to Type_handler::make_const_item_for_comparison()

This gives better performance, as excludes handler-by-type lookup.

This is a join patch fixing these two bugs:
MDEV-12560 Wrong data type for SELECT NULL UNION SELECT Point(1,1)
MDEV-12665 Hybrid functions do not preserve geometry type

Port of mysql changeset by Georgi Kodinov <Georgi.Kodinov@Oracle.com>:

    Bug #34325 : --add-drop-trigger option for mysqldump

    Implemented the --add-drop-trigger option to prepend each
    CREATE TRIGGER in the dump file with DROP TRIGGER.
    The option is off by default. Added a test case.

Fixing result set metadata for Item_int to match type_handler(),
i.e. MYSQL_TYPE_LONG for small numbers and MYSQL_TYPE_LONGLONG for big numbers.

The new 10.2 has log-slow-admin-statements ON but log-slow-queries OFF,
thus  there is no reason for a warning about it(the change was "by design")