Wait() option now accept *sync.WaitGroup

The original signature accept a boolean, and it feel like a little
verbose, since when people pass in this option, he/she always want to
pass a `true`.

Now if input `wg` is nil, it has same effect as passing `true` in
original code. Furthermore, if user want's finer grained control during
shutdown, one can pass in a predefined `wg`, so that server will wait
against it during shutdown.

function.go

@@ -54,7 +54,7 @@ func newFunction(opts ...Option) Function {
 
 	service.Server().Init(
 		// ensure the service waits for requests to finish
-		server.Wait(true),
+		server.Wait(nil),
 		// wrap handlers and subscribers to finish execution
 		server.WrapHandler(fnHandlerWrapper(fn)),
 		server.WrapSubscriber(fnSubWrapper(fn)),