Change SumOfDocumentFrequencies and SumOfTotalTermFrequencies to long (#4600)

This commit changes the SumOfDocumentFrequencies and SumOfTotalTermFrequencies
on TermVector to long. These values can be bigger than int.

Fixes #4578

(cherry picked from commit b4c1167)

Author: russcam

src/Nest/Document/Single/TermVectors/FieldStatistics.cs

@@ -9,9 +9,9 @@ public class FieldStatistics
 		public int DocumentCount { get; internal set; }
 
 		[DataMember(Name ="sum_doc_freq")]
-		public int SumOfDocumentFrequencies { get; internal set; }
+		public long SumOfDocumentFrequencies { get; internal set; }
 
 		[DataMember(Name ="sum_ttf")]
-		public int SumOfTotalTermFrequencies { get; internal set; }
+		public long SumOfTotalTermFrequencies { get; internal set; }
 	}
 }