Default socket close function.
Defined in header
template< class Protocol, class Executor> void beast_close_socket( net::basic_socket< Protocol, Executor >& sock);
This function is not meant to be called directly. Instead, it is called automatically
close_socket. To enable closure
of user-defined types or classes derived from a particular user-defined type,
this function should be overloaded in the corresponding namespace for the
type in question.