Directly reset callbacks without going through lwip functions (fix #30)

[mathieucarbou]

fix #30

[willmmiles]

At least as I understood the documentation, LwIP frees the tcp_pcb when _tcp_error returns. Any dereferencing of _pcb in AsyncClient::_error, or any other event callback after that point, is a use-after-free and will result in corrupting memory. This was the major impetus behind the redesign in #21.

[mathieucarbou]

At least as I understood the documentation, LwIP frees the tcp_pcb when _tcp_error returns. Any dereferencing of _pcb in AsyncClient::_error, or any other event callback after that point, is a use-after-free and will result in corrupting memory. This was the major impetus behind the redesign in #21.

@me-no-dev suggested also another approach I will try, which is to immediately cleanup and after send the event in the queue (for the user callback only)

[willmmiles]

You can skip most of the API cleanup as the pcb is about to be freed; none of the other callbacks will get called again anyways. The key cleanup is to invalidate the close_slot so that any in-flight operation on the async thread won't accidentally try to dereference the now-released pcb.