htlcswitch: fix mockInvoiceRegistry implementation of SettleInvoice
htlcswitch: fix mockInvoiceRegistry implementation of SettleInvoice

In this commit we fix the implementation of SettleInvoice by ensuring
the lock is held for the duration of the method.

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