* `NetworkStack::connect()`:
* Add timeout for connection attempt
* Now returns the socket at TCP ESTABLISHED or CLOSED states, or after connection timeout
* Split `NetworkStack::update()` into `update()` (for controlling the clock) and `poll()` (for polling the smoltcp EthernetInterface)
* Also remove option `auto_time_update`; the main application is responsible for what values `embedded_time::clock::Clock::try_now()` should return
Harry Ho
occheung