MB-18715 - moss Merge() didn't bump bufUsed correctly

And, also allocate more memory for both the partial and full merges.

index/store/moss/batch.go

@@ -54,6 +54,9 @@ func (b *Batch) Delete(key []byte) {
 }
 
 func (b *Batch) Merge(key, val []byte) {
+	if b.buf != nil {
+		b.bufUsed += len(key) + len(val)
+	}
 	b.merge.Merge(key, val)
 }
 

index/upside_down/upside_down.go

@@ -219,7 +219,7 @@ func (udc *UpsideDownCouch) batchRows(writer store.KVWriter, addRowsAll [][]Upsi
 	totBytes := addKeyBytes + addValBytes +
 		updateKeyBytes + updateValBytes +
 		deleteKeyBytes +
-		mergeKeyBytes + mergeValBytes
+		2*(mergeKeyBytes+mergeValBytes)
 
 	buf, wb, err := writer.NewBatchEx(store.KVBatchOptions{
 		TotalBytes: totBytes,