Fix for ticket #3020: set MySQL session time_zone variable to UTC ('+0:00') so...

Fix for ticket #3020: set MySQL session time_zone variable to UTC ('+0:00') so TIMESTAMP column values are comparable against our other UTC timestamp values.

MySQL stores TIMESTAMP columns as UTC, but with a local time interface. (SRSLY?!) DATETIME columns are always bare and assumed to be local time, but we keep only UTC in them.
Forcing the session time_zone to UTC means we won't have to worry as much about what we're sending/receiving in there.

Also will let us remove the hack in master commit a7abb232 for session tweaks

classes/Memcached_DataObject.php

@@ -480,6 +480,10 @@ class Memcached_DataObject extends Safe_DataObject
                     }
                 }
             }
+            // Needed to make timestamp values usefully comparable.
+            if (common_config('db', 'type') == 'mysql') {
+                parent::_query("set time_zone='+0:00'");
+            }
         }
 
         return $result;