invoices: optionally garbage collect invoices on the fly

This commit extends invoice garbage collection to also remove invoices
which are canceled when LND is already up and running. When the option
GcCanceledInvoicesOnTheFly is false (default) then invoices are kept and
the behavior is unchanged.

server.go

@@ -401,6 +401,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr, chanDB *channeldb.DB,
 		Clock:                       clock.NewDefaultClock(),
 		AcceptKeySend:               cfg.AcceptKeySend,
 		GcCanceledInvoicesOnStartup: cfg.GcCanceledInvoicesOnStartup,
+		GcCanceledInvoicesOnTheFly:  cfg.GcCanceledInvoicesOnTheFly,
 		KeysendHoldTime:             cfg.KeysendHoldTime,
 	}