tcp: stop timers and clean scoreboard when in tcp_close()
instead of doing that in tcp_discardcb(). A connection in
CLOSED state shall not need any timers. As the SACK scoreboard
is conceptually an extention of the socket buffer, remove
it when the socket buffer goes away with soisdisconnected().
Assert that no timer is rescheduled after that with tcp_timer_activate()
and verfiy that this is also the expected state in tcp_discardcb´().