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
