MDEV-23562 Assertion `time_type == MYSQL_TIMESTAMP_DATETIME' failed upon SELECT from versioned table

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.