htlcswitch: resolve invoice cancelation race condition
htlcswitch: resolve invoice cancelation race condition

Previously it could happen that an invoice was open at the time of the
LookupInvoice call, the htlc was settled because of that, but when the
SettleInvoice call was made eventually, it would fail because the
invoice was canceled in the mean time. The htlc would then be settled,
but the invoice not marked as such.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd