code: def get_name_value_hash_txid( self, name, value_hash ):
          
          rec = self.get_name( name )
          if rec is None:
              return None 
  
          if rec['revoked']:
              return None
          
          # find the txid of the given value hash
          if rec['value_hash'] == value_hash:
              return rec['txid']
  
          else:
              # search backwards for it 
              hist = rec['history']
              flat_hist = namedb_flatten_history( hist )
              for i in xrange(len(flat_hist)-1, 0, -1):
                  delta = flat_hist[i]
                  if delta['op'] == NAME_PREORDER:
                      # this name was re-registered. skip
                      return None 
  
                  if delta['value_hash'] == value_hash:
                      # this is the txid that affected it 
                      return delta['txid']
  
              # not found
              return None
docs: Given a name and a value hash (i.e. the zone file hash), return the txid for the value hash.
          Return None if the name doesn't exist, or is revoked, or did not
          receive a NAME_UPDATE since it was last preordered.