This algorithm allows a socket-like object to be closed in
a generic context. The customization point enables user-defined
types to define the close algorithm to use with `close_socket`.
fix#1417
* New get_lowest_layer free function works for any object
* New lowest_layer_type trait works for any object
* New examples and documentation on layered streams
API Changes:
* The member function lowest_layer is removed from all
types provided by the library:
Actions Required:
* Call the free function get_lowest_layer instead of
member lowest_layer.
