Add partial result to AggregateCursor continuation

[pengpeng-lu]

1. Add AggregateCursor.AggregateCursorContinuation, which contains partialAggregationResult.
2. AggregateCursor returns AggregateCursor.AggregateCursorContinuation
3. Add tests in record-layer and relational-layer to verify

[pengpeng-lu]

weird auto formating, will remove it in the next commit.

[alecgrieser]

I think this also needs to include the partialAggregationResult, though. I'm thinking of a case like:

try (RecordCursor<QueryResult> cursor = getAggregateCursor()) {
   RecordCursor<QueryResult> curr = cursor.getNext();
   while (curr.hasNext()) {
     // do something with curr
     curr = cursor.getNext();
   }

   RecordCursor<QueryResult> last = cursor.getNext();
   assert !last.hasNext(); // should succeed
   assert last.getContinution().equals(curr.getContinuation()); // I think will fail as last is missing any partial aggregation result 
}

The last value there is the second result from the cursor where .hasNext() is false.

[alecgrieser]

I don't see the partialAggregationResult at the moment. And did you add a test that called .getNext() a second time on a partially completed cursor?