multi: thread bandwidth check amount down to MayAddOutgoingHtlc

Pass htlc amount down to the channel so that we don't need to rely
on minHtlc (and pad it when the channel sets a 0 min htlc). Update
test to just check some sane values since we're no longer relying
on minHtlc amount at all.

htlcswitch/mock.go

@@ -784,7 +784,7 @@ func (f *mockChannelLink) Peer() lnpeer.Peer                            { return
 func (f *mockChannelLink) ChannelPoint() *wire.OutPoint                 { return &wire.OutPoint{} }
 func (f *mockChannelLink) Stop()                                        {}
 func (f *mockChannelLink) EligibleToForward() bool                      { return f.eligible }
-func (f *mockChannelLink) MayAddOutgoingHtlc() error                    { return nil }
+func (f *mockChannelLink) MayAddOutgoingHtlc(lnwire.MilliSatoshi) 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) {