net: tcp: Add retry handling

Adds a simple retry mechanism on top of the existing queueing
framework.

* On the first packet transmitted with an empty queue, the timer is
  started (to an initial timeout of 200ms).

* On a received, valid ACK, the timer is restarted.

* The timer is stopped whenever the last packet is ACKed and the queue
  becomes empty.

* On timer expiration, the timeout is doubled and the first
  unacknowledged packet is retransmitted.

* When receiving an ACK when we are known to be retransmitting, we
  mark all sent packets as needing to be sent again.

Change-Id: Iee3544ed43d0a2d50c29fa82f958a34cc10f7cda
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>