Members of nn_req which are related to an individual request split off.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
nn_task split from nn_req

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
nn_task_init and nn_task_term added

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>