Optional reference counting

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Instead of relying strictly on open/close events, a more dynamic resource management can be done with reference counting. Similar to OpenSSL, us_socket_up_ref could be added to ensure a different lifetime for the socket. This together with us_socket_is_closed makes it very useful in cases where you submit the socket to some async queue.

Cost of this could be 1 bit, 1 byte or 2 bytes. More than this, and something is just wrong.

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us_socket_is_closed is implemented

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Optional reference counting can always be implemented in a wrapper.