MDEV-16026: Forbid global system_versioning_asof in non-default time zone
* store `system_versioning_asof` in unix time;
* both session and global vars are processed in session timezone;
* setting `default` does not copy global variable anymore. Instead, it sets
system_time to SYSTEM_TIME_UNSPECIFIED, which means that no 'AS OF' time
is applied and `now()` can be assumed
As a regression, we cannot assign values below 1970 (UTC) anymore
MDEV-16481: set global system_versioning_asof=sf() crashes in specific case
* sys_vars.h: add `MYSQL_TIME` field to `set_var::save_result`
* sys_vars.ic: get rid of calling `var->value->get_date()` from
`Sys_var_vers_asof::update()`
* versioning.sysvars: add test; remove double warning
refactor Sys_var_vers_asof
* inherit from sys_var rather than Sys_var_enum
* remove junk "DEFAULT" keyword. There is DEFAULT in SQL grammar for it.
* make all conversions in check() to avoid possible errors
* avoid double var->value evaluation, which could
consequence in undefined behavior
The code in vers_select_conds_t::init_from_sysvar() assumed that
the value of the system_versioning_asof is DATETIME.
But it also could be DATE after a query like this:
SET system_versioning_asof = DATE(NOW());
Fixing Sys_var_vers_asof::update() to convert the value
of the assignment source to DATETIME if it returned DATE.
Now vers_select_conds_t::init_from_sysvar() always gets a DATETIME value.
Respect system fields in NO_ZERO_DATE mode.
This is the subject for refactoring in MDEV-19597
Conflict resolution from 7d5223310789f967106d86ce193ef31b315ecff0
Many related changes.
Note that AS OF condition must always be pushed down to physical tables,
it cannot be applied to a derived or a view. Thus:
* no versioning for internal temporary tables, they can never store
historical data.
* remove special versioning code from mysql_derived_prepare and
remove ER_VERS_DERIVED_PROHIBITED - derived can have no historical
data and cannot be prohibited for system versioning related reasons.
* do not expand select list for derived/views with sys vers fields,
derived/views can never have historical data.
* remove special invisiblity rules for sys vers fields, they are no
longer needed after the previous change
* remove system_versioning_hide, it lost the meaning after the
previous change.
* remove ER_VERS_SYSTEM_TIME_CLASH, it's no "clash", the inner
AS OF clause always wins.
* non-versioned fields in a historical query
reword the warning text, downgrade to note, don't
replace values with NULLs
