(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.
For now, that is correct.  I figured that we can just add those in as we come across use cases for them; they should be simple to add.  I've no doubt that we'll come up with use cases where we want/need them, but just thought it'd be best to get the basic framework in place first and then build on it (since this PR is kind of insanely huge already).

To me, the main goal of review of this PR is to affirm that in general this library swap seems like a sane idea.  If everyone's on board with that, there'll definitely end up being some polish PRs to follow over time.