In many places in Item::get_date() we use current_thd. After adding of MDEV-16991, access to THD will be needed at every call of get_date(), to check rounding mode from thd->variables.

To avoid heavy use of current_thd, we'll add a THD* parameter to Item::get_date().
Also, at the same time, we'll change "ulonglong fuzzydate" to "date_mode_t fuzzydate".

After this change, the definition of Item::get_date() will look like this:

  virtual bool get_date(THD *thd, MYSQL_TIME *ltime, date_mode_t fuzzydate)= 0;

Where date_mode_t:

• will have stricter data type control, so an attempt to pass to Item::get_date() an arbitrary integer value (instead of date_mode_t) won't compile
• will have a enum-based storage, so in gdb we can see easy readable values instead of integers:

  Breakpoint 1, Item_date_typecast::get_date (this=0x7ffe6c016158, thd=

  0x7ffe6c000b00, ltime=0x7ffff0c1d8f8, fuzzydate=...)

  at /home/bar/maria-git/server.10.4.get_date/sql/item_timefunc.cc:2452

  2452 date_mode_t tmp= (fuzzydate | sql_mode_for_dates(thd)) & ~TIME_TIME_ONLY;

  (gdb) p fuzzydate

  $1 = {

  m_mode = (date_mode_t::NO_ZERO_IN_DATE | date_mode_t::NO_ZERO_DATE | date_mode_t::INVALID_DATES)}

We'll also change a number of other dependency functions and methods to get "date_mode_t fuzzydate" instead of "ulonglong fuzzydate".