(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.
This might be a bit misleading if we end up following this pattern too often.  It's not obvious that this schema supports nil, and means we can't reuse it to force a non-nil callback function.  Also, I think it might be better anyways to limit nils in the system if at all possible, and it would seem like there would be a good "default" function to use in place of missing callback function.