I have some query logic that uses a stack of custom iterators with a
batch scanner.  The query begins to return values but then stops with a
'scanner closed' error.  The only reference I can find to scanner closed
in the src is in TabletServerBatchReaderIterator.  I can see that the
error is thrown when the query thread pool is shutdown, but I am not
sure why this is happening.  This query logic works on a single node
instance, but I get the scanner closed error when running on a multi-node

So far the stack traces have not been very helpful and we are not sure
where or how to troubleshoot this.  Any info on what conditions would
lead to a scanner closed error and where to begin looking to resolve
would be appreciated.