Missing thread_pool dtor implemented.
Revised thread pool implementation with std::thread-like submit method.
Added thread pool behaviour classes. Added spin_lock class.
Added nested class vector_stream in image_factory.
Refactored network thread state reporting.
Fixed erroneous range check.
Fixed dispatcher startup issue.
Added simple_message_base::body_after().
Added encode_to_stream and encode_to_vector methods in image_factory.
Resovled cyclic include.
