lnwallet: restore unsigned acked remote updates

This commit updates the channel state machine to
persistently store remote updates that we have received a
signature for, but that we haven't yet included in a commit
signature of our own.

Previously those updates were only stored in memory and
dropped across restarts. This lead to the production of
an invalid signature and channel force closure. The remote
party expects us to include those updates.

channeldb/db_test.go

@@ -403,7 +403,7 @@ func TestRestoreChannelShells(t *testing.T) {
 	// Ensure that it isn't possible to modify the commitment state machine
 	// of this restored channel.
 	channel := nodeChans[0]
-	err = channel.UpdateCommitment(nil)
+	err = channel.UpdateCommitment(nil, nil)
 	if err != ErrNoRestoredChannelMutation {
 		t.Fatalf("able to mutate restored channel")
 	}