add polymorphic_socket_base interface
add polymorphic_server_socket interface
add server_socket_adaptor to implement polymorphic_server_socket

This new class easily adapts classes fulfilling the Socket concept to
also implement the polymorphic_server_socket runtime-based polymorphic
interface.
add server_socket_reference_adaptor to implement polymorphic_server_socket

This new class easily adapts objects fulfilling the Socket concept to
also implement the polymorphic_server_socket runtime-based polymorphic
interface.