Add class 'task_manager' for managing tasks

Rename handlers:

urd::create_task      -> urd::iotask_create_handler
urd::check_on_task    -> urd::iotask_status_handler
urd::ping_request     -> urd::ping_handler
urd::register_job     -> urd::job_register_handler
urd::update_job       -> urd::job_update_handler
urd::remove_job       -> urd::job_remove_handler
urd::add_process      -> urd::process_add_handler
urd::remove_process   -> urd::process_remove_handler
urd::register_backend -> urd::backend_register_handler
urd::update_backend   -> urd::backend_update_handler
urd::remove_backend   -> urd::backend_remove_handler
urd::unknown_request  -> urd::unknown_request_handler