(TK-101) Move closing of http clients to a separate thread

This commit moves each close call made on a `CloseableHttpAsyncClient`
from the thread on which the request callback is made over to a
different thread in order to avoid a deadlock.

Previously, the client close call was made in the context of thread from
which the callback was invoked.  The Apache HTTP async client library
makes these calls from one of its i/o reactor threads.  In the case of a
"failed" request, e.g., failure to make the client connection, this call
occurs in the context of the primary i/o reactor thread.  The client
close call does an indefinite join on the primary i/o reactor thread
and, therefore, this call causes a deadlock.  The client never closes
and all resources associated with it - threads, file descriptors, etc. -
remain open until the process is shutdown.

This commit introduces a new class, `AsyncClose`, which manages a small,
fixed size thread pool from which client close calls can be executed.
The `async` Clojure and `JavaClient` APIs were updated to use `AsyncClose`
whenever a client needs to be closed from a callback.
Good call.  Ideally this shouldn't happen and makes sense for it to be more visible.  I'll re-throw this in a `RuntimeException`.