raft: make relationship between node and RawNode explicit

This will keep them from diverging to much. In fact we should remove
some of the obvious differences that have crept in over time so that
what remains is structural. This isn't done in this commit since
it amounts to a change in the public API; we should lump this in
when we break the public API the next time.

raft/rawnode.go

@@ -15,6 +15,7 @@
 package raft
 
 import (
+	"context"
 	"errors"
 
 	pb "go.etcd.io/etcd/raft/raftpb"
@@ -292,3 +293,44 @@ func (rn *RawNode) TransferLeader(transferee uint64) {
 func (rn *RawNode) ReadIndex(rctx []byte) {
 	_ = rn.raft.Step(pb.Message{Type: pb.MsgReadIndex, Entries: []pb.Entry{{Data: rctx}}})
 }
+
+// rawNodeAdapter is essentially a lint that makes sure that RawNode implements
+// "most of" Node. The exceptions (some of which are easy to fix) are listed
+// below.
+type rawNodeAdapter struct {
+	*RawNode
+}
+
+var _ Node = (*rawNodeAdapter)(nil)
+
+// Node specifies lead, which is pointless, can just be filled in.
+func (a *rawNodeAdapter) TransferLeadership(ctx context.Context, lead, transferee uint64) {
+	a.RawNode.TransferLeader(transferee)
+}
+
+// Node has a goroutine, RawNode doesn't need this.
+func (a *rawNodeAdapter) Stop() {}
+
+// RawNode returns a *Status.
+func (a *rawNodeAdapter) Status() Status { return *a.RawNode.Status() }
+
+// RawNode takes a Ready. It doesn't really have to do that I think? It can hold on
+// to it internally. But maybe that approach is frail.
+func (a *rawNodeAdapter) Advance() { a.RawNode.Advance(Ready{}) }
+
+// RawNode returns a Ready, not a chan of one.
+func (a *rawNodeAdapter) Ready() <-chan Ready { return nil }
+
+// Node takes more contexts. Easy enough to fix.
+
+func (a *rawNodeAdapter) Campaign(context.Context) error { return a.RawNode.Campaign() }
+func (a *rawNodeAdapter) ReadIndex(_ context.Context, rctx []byte) error {
+	a.RawNode.ReadIndex(rctx)
+	// RawNode swallowed the error in ReadIndex, it probably should not do that.
+	return nil
+}
+func (a *rawNodeAdapter) Step(_ context.Context, m pb.Message) error   { return a.RawNode.Step(m) }
+func (a *rawNodeAdapter) Propose(_ context.Context, data []byte) error { return a.RawNode.Propose(data) }
+func (a *rawNodeAdapter) ProposeConfChange(_ context.Context, cc pb.ConfChange) error {
+	return a.RawNode.ProposeConfChange(cc)
+}