raft: do not reset vote if term is not changed

raft MUST keep the voting information for the same term. reset
should not reset vote if term is not changed.

raft/multinode_test.go

@@ -255,7 +255,7 @@ func TestMultiNodeStart(t *testing.T) {
 	wants := []Ready{
 		{
 			SoftState: &SoftState{Lead: 1, RaftState: StateLeader},
-			HardState: raftpb.HardState{Term: 2, Commit: 2},
+			HardState: raftpb.HardState{Term: 2, Commit: 2, Vote: 1},
 			Entries: []raftpb.Entry{
 				{Type: raftpb.EntryConfChange, Term: 1, Index: 1, Data: ccdata},
 				{Term: 2, Index: 2},
@@ -266,7 +266,7 @@ func TestMultiNodeStart(t *testing.T) {
 			},
 		},
 		{
-			HardState:        raftpb.HardState{Term: 2, Commit: 3},
+			HardState:        raftpb.HardState{Term: 2, Commit: 3, Vote: 1},
 			Entries:          []raftpb.Entry{{Term: 2, Index: 3, Data: []byte("foo")}},
 			CommittedEntries: []raftpb.Entry{{Term: 2, Index: 3, Data: []byte("foo")}},
 		},

raft/node_test.go

@@ -304,7 +304,7 @@ func TestNodeStart(t *testing.T) {
 	wants := []Ready{
 		{
 			SoftState: &SoftState{Lead: 1, RaftState: StateLeader},
-			HardState: raftpb.HardState{Term: 2, Commit: 2},
+			HardState: raftpb.HardState{Term: 2, Commit: 2, Vote: 1},
 			Entries: []raftpb.Entry{
 				{Type: raftpb.EntryConfChange, Term: 1, Index: 1, Data: ccdata},
 				{Term: 2, Index: 2},
@@ -315,7 +315,7 @@ func TestNodeStart(t *testing.T) {
 			},
 		},
 		{
-			HardState:        raftpb.HardState{Term: 2, Commit: 3},
+			HardState:        raftpb.HardState{Term: 2, Commit: 3, Vote: 1},
 			Entries:          []raftpb.Entry{{Term: 2, Index: 3, Data: []byte("foo")}},
 			CommittedEntries: []raftpb.Entry{{Term: 2, Index: 3, Data: []byte("foo")}},
 		},

raft/raft.go

@@ -358,9 +358,11 @@ func (r *raft) maybeCommit() bool {
 }
 
 func (r *raft) reset(term uint64) {
-	r.Term = term
+	if r.Term != term {
+		r.Term = term
+		r.Vote = None
+	}
 	r.lead = None
-	r.Vote = None
 	r.elapsed = 0
 	r.votes = make(map[uint64]bool)
 	for i := range r.prs {