tcp: Rework segment arrival handling

Follow RFC793 section "SEGMENT ARRIVES".

There are 4 major cases:

1) If the state is CLOSED
2) If the state is LISTEN
3) If the state is SYN-SENT
4) If the state is other state

Note:

- This change is significant (more than 10 pages in RFC793 describing
  this segment arrival handling).
- More test is needed. Good news is, so far, tcp_server(ping/txtx/rxrx)
  tests and httpd work fine.