Skip adding operations without sequence number

A version conflict exception can happen during recovery. If this
operation is from an old primary, a sequence number will have not been
assigned to the operation. In this case, we should skip adding a no-op
to the translog.

Author: jasontedor

core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -671,7 +671,7 @@ private IndexResult innerIndex(Index index) throws IOException {
             if (checkVersionConflictResult.isPresent()) {
                 indexResult = checkVersionConflictResult.get();
                 // norelease: this is not correct as this does not force an fsync, and we need to handle failures including replication
-                if (indexResult.hasFailure()) {
+                if (indexResult.hasFailure() || seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO) {
                     location = null;
                 } else {
                     final Translog.NoOp operation = new Translog.NoOp(seqNo, index.primaryTerm(), "version conflict during recovery");
@@ -809,7 +809,7 @@ private DeleteResult innerDelete(Delete delete) throws IOException {
             if (result.isPresent()) {
                 deleteResult = result.get();
                 // norelease: this is not correct as this does not force an fsync, and we need to handle failures including replication
-                if (deleteResult.hasFailure()) {
+                if (deleteResult.hasFailure() || seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO) {
                     location = null;
                 } else {
                     final Translog.NoOp operation = new Translog.NoOp(seqNo, delete.primaryTerm(), "version conflict during recovery");