This commit removes the `AsyncClose` class and references to it. These
changes are in preparation for making the close call after the response
has been dereferenced - to be done in a later commit. The `persistent`
flag/option was also removed from request handling as it will no longer
be used for determining whether or not the client should be closed
following a request.
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.
Prior to this commit, the Java client API would throw an NPE if
it got a response from a server where the Content-Type header
did not include a charset.
