Move-assign a basic_socket_acceptor
from another.
basic_socket_acceptor & operator=( basic_socket_acceptor && other);
This assignment operator moves an acceptor from one object to another.
The other basic_socket_acceptor
object from which the move will occur.
Following the move, the moved-from object is in the same state as if
constructed using the basic_socket_acceptor(const executor_type&)
constructor.