Investigate whether HTTP handlers are all correctly async

[andrewazores]

#310 (review)

The isAsync method on RequestHandlers returns true by default, but some handlers override this to return false. This value is used by the WebServer to determine how to install the handler into the Router - if isAsync == true then the handler is installed normally, otherwise it is installed as a blocking handler. Any handlers which are not themselves implemented async should have isAsync == false so that Vertx runs the handler on a separate worker thread (perhaps with some small exceptions like GrafanaDashboardUrlGetHandler which simply returns a String read from env).

The goal in investigating/fixing this issue is not to reimplement handlers to add/remove async pieces of their implementations but simply to check behaviours and update isAsync to match.