invoices: optionally hold and auto-cancel keysend payments

Adds a new configuration flag to lnd that will keep keysend payments in
the accepted state. An application can then inspect the payment
parameters and decide whether to settle or cancel.

The on-the-fly inserted keysend invoices get a configurable expiry time.
This is a safeguard in case the application that should decide on the
keysend payments isn't active.

server.go

@@ -417,6 +417,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr, chanDB *channeldb.DB,
 		HtlcHoldDuration:     invoices.DefaultHtlcHoldDuration,
 		Clock:                clock.NewDefaultClock(),
 		AcceptKeySend:        cfg.AcceptKeySend,
+		KeysendHoldTime:      cfg.KeysendHoldTime,
 	}
 
 	s := &server{