httpContext in ApacheHttpClient4Executor might not be thread safe

ApacheHttpClient4Engine keeps the httpContext as member variable. This makes sense in some cases but wondering if this would be thread safe on other cases where data in the context gets thrown on a per request.

In the case where the client is pooled the same client engine is serving mutiple requests concurrently. There might be cases where the httpcontext sets data per request (per thread)

Not sure if this would really happen but just wanted to throw the defect out there.