Remove usage of non thread-safe BasicHttpContext

While doing some highly concurrent CouchDB requests using LightCouch I encountered exceptions in apache http client (ConnectionShutdownException).

After modifying the code to create BasicHttpContext for each request the problem seems to go away (although I can't be entirely sure since the errors seemed to have occurred randomly).

Either way, BasicHttpContext class is marked as being not thread safe therefore this change should be done anyway.