RAFT node responds to VoteRequest with outdated Term (#5021)

A node's Term may be increased during handling of a VoteRequest. When
that was the case, the node was responding with a stale Term.

Example trace:

```
[DBG] RAFT [S1Nunr6R - S-R3F-5xgoWjQW] Received a voteRequest &{term:27 lastTerm:26 lastIndex:46 candidate:yrzKKRBu reply:$NRG.R.OHF7VRUO}
[DBG] RAFT [S1Nunr6R - S-R3F-5xgoWjQW] Stepping down from leader, detected higher term: 27 vs 26
[DBG] RAFT [S1Nunr6R - S-R3F-5xgoWjQW] Stepping down
[DBG] RAFT [S1Nunr6R - S-R3F-5xgoWjQW] Switching to follower
[DBG] RAFT [S1Nunr6R - S-R3F-5xgoWjQW] Sending a voteResponse &{term:26 peer:S1Nunr6R granted:true} -> "$NRG.R.OHF7VRUO"
```
This is handled correctly except VoteResponse in the last log line
should be sending back Term 27, not 26.

Signed-off-by: Your Name <marco@nats.io>

server/raft.go

@@ -3953,6 +3953,10 @@ func (n *raft) processVoteRequest(vr *voteRequest) error {
 			n.resetElect(randCampaignTimeout())
 		}
 	}
+
+	// Term might have changed, make sure response has the most current
+	vresp.term = n.term
+
 	n.Unlock()
 
 	n.sendReply(vr.reply, vresp.encode())

server/raft_test.go

@@ -273,15 +273,13 @@ func TestNRGSimpleElection(t *testing.T) {
 		re := decodeVoteResponse(msg.Data)
 		require_True(t, re != nil)
 
-		// The new term hasn't started yet, so the vote responses
-		// should contain the term from before the election. It is
-		// possible that candidates are listening to this to work
-		// out if they are in previous terms.
-		require_Equal(t, re.term, vr.lastTerm)
-		require_Equal(t, re.term, startTerm)
-
 		// The vote should have been granted.
 		require_Equal(t, re.granted, true)
+
+		// The node granted the vote, therefore the term in the vote
+		// response should have advanced as well.
+		require_Equal(t, re.term, vr.term)
+		require_Equal(t, re.term, startTerm+1)
 	}
 
 	// Everyone in the group should have voted for our candidate