net: tcp2: Add send_data queue into the TCP connection

In order to support the retransmission for the outgoind data,
add the following into the TCP connection:

- struct net_pkt *send_data (used as a queue)
- send_data_total
- unack_len
- send_data_timer
- send_data_retries

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