(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.
There was no testing at all before on the async side; I just wanted to get *some* coverage.  The sync side is implemented by calling through to the async side so I feel like the sync tests are testing both for now.  Does that seem OK?  If we were ever to rework the sync side so that it didn't call through to the async under the covers, then I'd think we'd need to duplicate all of the tests...