[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_thumbnail.php

@@ -33,7 +33,7 @@ class File_thumbnail extends Managed_DataObject
     public $filename;                        // text
     public $width;                           // int(4)  primary_key
     public $height;                          // int(4)  primary_key
-    public $modified;                        // datetime()   not_null default_CURRENT_TIMESTAMP
+    public $modified;                        // timestamp() not_null default_CURRENT_TIMESTAMP
 
     const URLHASH_ALG = 'sha256';
 
@@ -47,7 +47,7 @@ class File_thumbnail extends Managed_DataObject
                 'filename' => array('type' => 'text', 'description' => 'if stored locally, filename is put here'),
                 'width' => array('type' => 'int', 'not null' => true, 'description' => 'width of thumbnail'),
                 'height' => array('type' => 'int', 'not null' => true, 'description' => 'height of thumbnail'),
-                '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('file_id', 'width', 'height'),
             'indexes' => array(