kvserver: use Step for conf changes as well

ProposeConfChange just calls MarshalConfChange followed by Step, and
by inlining this in our codebase we gain the ability to figure out if
raft rejected our ConfChange. We will need this in a follow-up commit.

pkg/kv/kvserver/replica_proposal_buf.go

@@ -203,7 +203,6 @@ type proposerRaft interface {
 	Step(raftpb.Message) error
 	Status() raft.Status
 	BasicStatus() raft.BasicStatus
-	ProposeConfChange(raftpb.ConfChangeI) error
 	Campaign() error
 }
 
@@ -563,9 +562,18 @@ func (b *propBuf) FlushLockedWithRaftGroup(
 				continue
 			}
 
-			if err := raftGroup.ProposeConfChange(
-				cc,
-			); err != nil && !errors.Is(err, raft.ErrProposalDropped) {
+			typ, data, err := raftpb.MarshalConfChange(cc)
+			if err != nil {
+				firstErr = err
+				continue
+			}
+			msg := raftpb.Message{Type: raftpb.MsgProp, Entries: []raftpb.Entry{
+				{
+					Type: typ,
+					Data: data,
+				},
+			}}
+			if err := raftGroup.Step(msg); err != nil && !errors.Is(err, raft.ErrProposalDropped) {
 				// Silently ignore dropped proposals (they were always silently
 				// ignored prior to the introduction of ErrProposalDropped).
 				// TODO(bdarnell): Handle ErrProposalDropped better.

pkg/kv/kvserver/replica_proposal_buf_test.go

@@ -126,11 +126,6 @@ func (t testProposerRaft) BasicStatus() raft.BasicStatus {
 	return t.status.BasicStatus
 }
 
-func (t testProposerRaft) ProposeConfChange(i raftpb.ConfChangeI) error {
-	// TODO(andrei, nvanbenschoten): Capture the message and test against it.
-	return nil
-}
-
 func (t *testProposerRaft) Campaign() error {
 	t.campaigned = true
 	return nil