(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.
I don't think it's worth changing in this PR, or even if it's the right thing to do in this case. I was mainly saying that I think we should't always necessarily put the `maybe` part in the schema itself because it means we can't re-use the actual Fn part of it without getting the Nil part too. Minor point, and just more of food for thought than a prescription.