net: tcp: Rework ACK behavior

TCP ACK values aren't known until packet transmission time.  Set the
ACK value then, logging the last one sent for later comparison.

Note that this allows us to effect a neat trick: if your
net_context_send() callback produces a synchronous reply on the
stream, Zephyr can include the ACK in the sent data and not have to
transmit a separate packet.  Traditional OSes with internal queing and
a syscall interface can't do this.

Change-Id: If8b8546dac6163d763b8f8dd75865192f2eb302a
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>