Avoid the use of deprecated MariaDB "zero dates" globally. If they're present
as attribute defaults somewhere, they will be replaced with NULL implicitly.
The existing "zero dates" in MariaDB storage will be left intact and this
should not present any issues.
The "timestamp" type in table definitions now corresponds to DATETIME in
MariaDB with "DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP", which
should be close enough to the original behaviour for compatibility purposes.
It is now the recommended type for "modified" attributes, because of the
update trigger on MariaDB. But there is no such trigger implemented on
PostgreSQL as of this moment.
Memcached_DataObject now defines
* pkeyGetClass to avoid collision with Managed_DataObject pkeyGet
* getClassKV to avoid collision with Managed_DataObject getKV
The switch to having a function in Managed_DataObject is now being applied
to plugins which can use this as well.
There are some plugins that still use Memcached_DataObject, but these will
be taken care of later.
