New AsyncHttp4sServlet builder doesn't have `serviceErrorHandler`

[dhinojosa]

AsyncHttp4sServlet from a discussion with Ross seems to be missing serviceErrorHandler. Not sure if it should be included or not, but I added a ticket just in case.

Here is what methods are included currently.

AsyncHttp4sServlet
          .builder(httpApp, dispatcher)
          .withAsyncTimeout(builder.asyncTimeout)
          .withChunkSize(chunkSize)
          .build

[rossabaker]

I argue it shouldn't: a middleware achieves the same end, and applies to all httpApp (it's just functions!) instead of needing to be maintained on each backend.