(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.
Yeah, I hate this.  And I remember that we had this conversation a while back on a PR that you submitted.

Since it seems like there's *no* way to express this without having some level of dependence on the language, I thought I'd just pick a single interface, to minimize the surface area.  So then it became a matter of trying to decide which of those interfaces most closely mapped to what we're using this for, and `IBlockingDeref` seemed the most sensible to me.

I think I'm still a proponent of only choosing a single interface, but I'd be find switching the chosen interface to one of the other ones if you think any of them make more sense.

Do you remember where we landed the last time this came up?