Follow-up to 49b921b.
Move the duplicated addr_map lookup into detail::apply_addr_map, shared by
ClientImpl::create_client_socket and WebSocketClient::connect.
Add a WebSocketClient test for a hostname mapped value, so that path has
the same coverage as the Client one. Guard its teardown with scope_exit:
a failing ASSERT_TRUE returns from the test body, and destroying a still
joinable std::thread calls std::terminate, taking the whole binary down.
Document set_hostname_addr_map in README. It had no entry at all.
ae8356d86e
