Add more network statistics

Add a new struct `ntb_network_stats`. 
We now keep track of all data send and recieved
Add connect and disconnect to ntb_network

As most mobile data plans have only a limited amount of fast mobile data, we
want to be able to shutdown all connections when we are not connected via wlan.

On a disconnect ntb_network sets the boolean member `sleep` of 
ntb_main_context to true. ntb_main_context will then wait on the `sleep_cond`
 pthread condition variable until an other thread will awake it.