[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/File_redirection.php

@@ -30,7 +30,7 @@ class File_redirection extends Managed_DataObject
     public $file_id;                         // int(4)
     public $redirections;                    // int(4)
     public $httpcode;                        // int(4)
-    public $modified;                        // datetime()   not_null default_CURRENT_TIMESTAMP
+    public $modified;                        // timestamp() not_null default_CURRENT_TIMESTAMP
 
     /* the code above is auto generated do not remove the tag below */
     ###END_AUTOCODE
@@ -46,7 +46,7 @@ class File_redirection extends Managed_DataObject
                 'file_id' => array('type' => 'int', 'description' => 'short URL for what URL/file'),
                 'redirections' => array('type' => 'int', 'description' => 'redirect count'),
                 'httpcode' => array('type' => 'int', 'description' => 'HTTP status code (20x, 30x, etc.)'),
-                'modified' => array('type' => 'datetime', 'not null' => true, 'default' => 'CURRENT_TIMESTAMP', 'description' => 'date this record was modified'),
+                'modified' => array('type' => 'timestamp', 'not null' => true, 'description' => 'date this record was modified'),
             ),
             'primary key' => array('urlhash'),
             'foreign keys' => array(