scorch optimizations for mem.PostingsIterator.Next() & docTermMap

Due to the usage rules of iterators, mem.PostingsIterator.Next() can
reuse its returned Postings instance.

Also, there's a micro optimization in persistDocValues() for one fewer
access to the docTermMap in the inner-loop.

index/scorch/segment/mem/posting.go

@@ -73,6 +73,7 @@ type PostingsIterator struct {
 	offset    int
 	locoffset int
 	actual    roaring.IntIterable
+	reuse     Posting
 }
 
 // Next returns the next posting on the postings list, or nil at the end
@@ -92,17 +93,16 @@ func (i *PostingsIterator) Next() (segment.Posting, error) {
 		i.offset++
 		allN = i.all.Next()
 	}
-	rv := &Posting{
+	i.reuse = Posting{
 		iterator:  i,
 		docNum:    uint64(n),
 		offset:    i.offset,
 		locoffset: i.locoffset,
 		hasLoc:    i.locations.Contains(n),
 	}
-
 	i.locoffset += int(i.postings.dictionary.segment.Freqs[i.postings.postingsID-1][i.offset])
 	i.offset++
-	return rv, nil
+	return &i.reuse, nil
 }
 
 // Posting is a single entry in a postings list

index/scorch/segment/zap/build.go

@@ -552,8 +552,7 @@ func persistDocValues(memSegment *mem.Segment, w *CountHashWriter,
 			nextPosting, err2 := postingsItr.Next()
 			for err2 == nil && nextPosting != nil {
 				docNum := nextPosting.Number()
-				docTermMap[docNum] = append(docTermMap[docNum], []byte(next.Term)...)
-				docTermMap[docNum] = append(docTermMap[docNum], termSeparator)
+				docTermMap[docNum] = append(append(docTermMap[docNum], []byte(next.Term)...), termSeparator)
 				nextPosting, err2 = postingsItr.Next()
 			}
 			if err2 != nil {
@@ -562,10 +561,10 @@ func persistDocValues(memSegment *mem.Segment, w *CountHashWriter,
 
 			next, err = dictItr.Next()
 		}
-
 		if err != nil {
 			return nil, err
 		}
+
 		// sort wrt to docIDs
 		var docNumbers docIDRange
 		for k := range docTermMap {