After looking at the BIND 9 code more closely, I found we can complete this
task with the advanced feature such as getting "next node".  So I'm
incorporating changes in #517 that are necessary for this task:
 - the 'return empty node' swith
 - revised interface with 'node chain' (we don't need a chain for this task,
   but we'll need an additional parameter for find() to record some details
   about the search)

The incorporated part has been already reviewed, so this change
doesn't have to be reviewed explicitly.