Fix #757 (ReferenceError in TangoAttrValue)
TangoAttrValue keeps a weak reference to its associated TangoAttribute, which is used for accessing the "configuration" API. But, unless the TangoAttribute is hard-referenced elsewhere, the weakref would be broken. This problem has been hidden due to issue #511 (closed), but it is evident now since we fixed it in #692. Fix #757 (closed) by recreating the attribute when needed.