- Added mem_root to all calls to new Item
- Added private method operator new(size_t size) to Item to ensure that
we always use a mem_root when creating an item.
This saves use once call to current_thd per Item creation
Added mandatory thd parameter to Item (and all derivative classes) constructor.
Added thd parameter to all routines that may create items.
Also removed "current_thd" from Item::Item. This reduced number of
pthread_getspecific() calls from 290 to 177 per OLTP RO transaction.
MDEV-6666 Malformed result for CONCAT(utf8_column, binary_string)
Item_static_string_func::safe_charset_converter() and
Item_hex_string::safe_charset_converter() did not
handle character sets with mbminlen>1 properly, as well as
did not handle conversion from binary to multi-byte well.
Introducing Item::const_charset_converter(), to reuse it in a number
of Item_*::safe_charset_converter().
