net: tcp2: Implement the retransmission for the outgoing data

In order to support the retransmission for the outgoing data:

1. The outgoing data packet is appended to the send_data queue
   in net_tcp_queue_data().

2. tcp_send_queued_data() is called and will use tcp_send_data()
   to sends queued but unsent data packet by packet
   until there's an unsent data and the receiver's window isn't full.

   tcp_send_queued_data() subscribes send_data_timer
   that will handle retrasmissions with tcp_resend_data().

3. tcp_send_data() peeks a single chunk of data from the send_data
   queue that will not exceed the maximum segment size
   until the the receiver's window is full.

   tcp_send_data() uses conn->seq and conn->unack_len as the sequence
   number for the TCP packet.

   conn->unacked_len is advanced on each send.

4. On data acknowledgment:

   - acknowledged amount of data is removed from the beginning
     of the send_data queue
   - conn->seq is advanced by the acknowledged amount
   - conn->unacked_len is decremented by the acknowledged amount
   - send_data_timer is cancelled
   - tcp_send_queued_data() is called to send queued but
     prevoiusly unsent data

5. On timeout, tcp_resend_data() will reset conn->unack_len,
   peek one packet from the beginning of the send_queue and resend,
   terminating the connection on retries exceeded.

   Meanwhile the outgoing data tcp_send_queued_data() is just
   appended to the send_data but not sent.

   In case of the acknowledgement, tcp_send_queued_data() will
   start sending multiple packets until the receiver's window
   is full.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>