Add nocommit to sequence number stats method

This commit adds a nocommit to the SeqNoFieldType#stats method as the
implementation is temporary until late-binding commits are available.

Author: jasontedor

core/src/main/java/org/elasticsearch/index/mapper/internal/SeqNoFieldMapper.java

@@ -116,6 +116,8 @@ public Query termQuery(Object value, @Nullable QueryShardContext context) {
 
         @Override
         public FieldStats stats(IndexReader reader) throws IOException {
+            // nocommit remove implementation when late-binding commits
+            // are possible
             final List<LeafReaderContext> leaves = reader.leaves();
             if (leaves.isEmpty()) {
                 return null;