In some multithread cases there is possibility that process_oob function
was called after ATHandler was deleted. Fix is to wait if oob processing
is ongoing.
CellularContext now tries to get an IP address after connect and before
sending NSAPI_STATUS_GLOBAL_UP. Even if we don't the IP address from
the modem we will send NSAPI_STATUS_GLOBAL_UP and return success.
Modem has an ip address but for some reason some modems don't
give it to us.