AthContainers: Add AuxVectorData::lockDecoration.
Add AuxVectorData::lockDecoration to explicitly lock a decoration. This is only safe as long as no other thread can be accessing this decoration. Therefore, it is preferable to do this as a side-effect of using WriteDecorHandle. This can be used if that is not possible. However, this is a non-const method as a reminder.
See ATEAM-909.