Feeders push checkpoints even if they are equivalent

The feeder previously short circuited if the witness had already witnessed an equivalent checkpoint. This meant that logs would get stale timestamps from the witnesses. This change makes it so that the witness will keep updating their timestamps on checkpoints.

internal/feeder/feeder.go

@@ -150,7 +150,11 @@ func submitToWitness(ctx context.Context, cpRaw []byte, cpSubmit log.Checkpoint,
 			}
 			if latestCP.Size == cpSubmit.Size && bytes.Equal(latestCP.Hash, cpSubmit.Hash) {
 				klog.V(1).Infof("%q unchanged - @%d: %x", logName, latestCP.Size, latestCP.Hash)
-				returnCp = latestCPRaw
+				if returnCp, err = opts.Witness.Update(ctx, opts.LogID, cpRaw, [][]byte{}); err != nil {
+					e := fmt.Errorf("failed to submit fresh checkpoint to witness: %v", err)
+					klog.Warning(e.Error())
+					return e
+				}
 				return nil
 			}
 		}