[SCHEMA] Improve timestamp storage

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.

classes/Group_join_queue.php

@@ -14,12 +14,11 @@
 // You should have received a copy of the GNU Affero General Public License
 // along with GNU social.  If not, see <http://www.gnu.org/licenses/>.
 
-defined('GNUSOCIAL') || die();
-
 /**
  * Table Definition for request_queue
  */
-require_once INSTALLDIR.'/classes/Memcached_DataObject.php';
+
+defined('GNUSOCIAL') || die();
 
 class Group_join_queue extends Managed_DataObject
 {
@@ -41,7 +40,7 @@ class Group_join_queue extends Managed_DataObject
             'fields' => array(
                 'profile_id' => array('type' => 'int', 'not null' => true, 'description' => 'remote or local profile making the request'),
                 'group_id' => array('type' => 'int', 'not null' => true, 'description' => 'remote or local group to join, if any'),
-                'created' => array('type' => 'datetime', 'not null' => true, 'default' => '0000-00-00 00:00:00', 'description' => 'date this record was created'),
+                'created' => array('type' => 'datetime', 'description' => 'date this record was created'),
             ),
             'primary key' => array('profile_id', 'group_id'),
             'indexes' => array(