htlcswitch: extend ChannelLink interface with ShutdownIfChannelClean

This allows a caller to ensure to optimistically shut down the link
if the channel is clean. If the channel is not clean, an error is
returned and the link continues functioning as normal. The caller
should also call RemoveLink to ensure that the link isn't seen as
usable within the switch.

htlcswitch/mock.go

@@ -757,6 +757,7 @@ func (f *mockChannelLink) ChannelPoint() *wire.OutPoint                 { return
 func (f *mockChannelLink) Stop()                                        {}
 func (f *mockChannelLink) EligibleToForward() bool                      { return f.eligible }
 func (f *mockChannelLink) MayAddOutgoingHtlc() error                    { return nil }
+func (f *mockChannelLink) ShutdownIfChannelClean() error                { return nil }
 func (f *mockChannelLink) setLiveShortChanID(sid lnwire.ShortChannelID) { f.shortChanID = sid }
 func (f *mockChannelLink) UpdateShortChanID() (lnwire.ShortChannelID, error) {
 	f.eligible = true