Clearing the m_listeners map when it becomes empty

[IvanEsyagin-iss]

http_linux_server has a map std::map<std::string, std::unique_ptr<hostport_listener>, iequal_to> m_listeners, where new hostport_listener are added with the host+port key in the pplx::task http_server_api::register_listener() function. But there is no deletion from this map in the pplx::task http_linux_server::unregister_listener() function. Therefore, until http_linux_server is deleted(that is, we will not close all open listener) the map will contain an hostport_listener that has already been created.