lnrpc+rpc+lnd: add new Stop command for gracefully shutting down lnd

This commit adds simple non-blocking stop command to lncli, with an
appropriate proto update and implementation within the rpcserver.  When
invoked the interrupt handler routine in signal.go with begin the graceful
shutdown of lnd.

cmd/lncli/main.go

@@ -79,6 +79,7 @@ func main() {
 		debugLevelCommand,
 		decodePayReqComamnd,
 		listChainTxnsCommand,
+		stopCommand,
 	}
 
 	if err := app.Run(os.Args); err != nil {