scorch zap allocate govarint.U64Base128Encoder just once

Instead of allocating a govarint.U64Base128Encoder in the inner loop,
allocate it just once on the outside, as it appears that it's just a
thin wrapper around binary.PutUvarint().

index/scorch/segment/zap/merge.go

@@ -390,6 +390,8 @@ func mergeStoredAndRemap(segments []*Segment, drops []*roaring.Bitmap,
 	var metaBuf bytes.Buffer
 	var data, compressed []byte
 
+	metaEncoder := govarint.NewU64Base128Encoder(&metaBuf)
+
 	vals := make([][][]byte, len(fieldsInv))
 	typs := make([][]byte, len(fieldsInv))
 	poss := make([][][]uint64, len(fieldsInv))
@@ -407,8 +409,6 @@ func mergeStoredAndRemap(segments []*Segment, drops []*roaring.Bitmap,
 			compressed = compressed[:0]
 			curr = 0
 
-			metaEncoder := govarint.NewU64Base128Encoder(&metaBuf)
-
 			if drops[segI] != nil && drops[segI].Contains(uint32(docNum)) {
 				segNewDocNums = append(segNewDocNums, docDropped)
 			} else {