(TK-23) Port to apache HttpAsyncClient

* Ports the code over to use the Apache HttpAsyncClient library
  instead of http-kit, as we were getting some weird and inconsistent
  SSL errors from http-kit, and
* Simplify the API (by eliminating a lot of the supported configuration
  options).  Some of these options we'll eventually want to add back in,
  but for now, getting rid of them makes the API less cluttered and
  also gives us the opportunity to do a better job writing tests
  for new options when we do add support for them.

For now it only supports constructing a new client on every request.
I intend to add API for creating a client explicitly, and a protocol
for how to interact with the client.  This will allow users to
re-use an existing client as they see fit... but this hasn't been
implemented yet.
One of @nwolfe's points, I think, is that the `maybe` is not obvious from the name.  Some names might indicate that something could be `nil`, but since this is a 'fn' it seems like it would never be `nil`.

I don't think that extracting the `maybe` is the worst thing, and that's probably what I'd do.  Another option would be to just rename this to something like `MaybeResponseCallbackFn`, but that's kinda icky.  This is a minor point to me, so I'm fine with whatever is decided.