net: tcp: Change SYN FIN to use send_data_timer

The send_queue was used as SYN/FIN packet retransmission. Before
the SYN/FIN being ACKed and dequeue-ed, the following packets in
the send_queue cannot be sent out. That's why Zephyr had to send
a FIN+ACK instead of a duplicated ACK-only in FINWAIT1, CLOSING.
In fact, we can take SYN/FIN as kind of data and use the same
send_data_timer for retransmission, like other OSes do. This way,
the send_queue is simply used for local traffics.
Benefits (in theory):
1. The code is easier,
2. TxPkt performance is better after skipping enq/deq send_queue,
3. The struct tcp{} node is a few bytes smaller, saving memory.

Signed-off-by: Shrek Wang <inet_eman@outlook.com>