contrib/raftexample: Save snapshot and WAL before hard state

Update raftexample to save the snapshot file and WAL snapshot entry
before hardstate to ensure the snapshot exists during recovery.
Otherwise if there is a failure after storing the hard state there may
be reference to a non-existent snapshot.
This PR introduces the fix from etcd-io#10219 to the raftexample.

contrib/raftexample/raft.go

@@ -449,9 +449,13 @@ func (rc *raftNode) serveChannels() {
 
 		// store raft entries to wal, then publish over commit channel
 		case rd := <-rc.node.Ready():
-			rc.wal.Save(rd.HardState, rd.Entries)
+			// Must save the snapshot file and WAL snapshot entry before saving any other entries
+			// or hardstate to ensure that recovery after a snapshot restore is possible.
 			if !raft.IsEmptySnap(rd.Snapshot) {
 				rc.saveSnap(rd.Snapshot)
+			}
+			rc.wal.Save(rd.HardState, rd.Entries)
+			if !raft.IsEmptySnap(rd.Snapshot) {
 				rc.raftStorage.ApplySnapshot(rd.Snapshot)
 				rc.publishSnapshot(rd.Snapshot)
 			}