tcp: stop timers and clean scoreboard in tcp_close()

tcp: stop timers and clean scoreboard in tcp_close()

Stop timers when in tcp_close() instead of doing that in tcp_discardcb().
A connection in CLOSED state shall not need any timers. Assert that no
timer is rescheduled after that in tcp_timer_activate() and verfiy that
this is also the expected state in tcp_discardcb().

PR:			276761
Reviewed By:		glebius, tuexen, #transport
Sponsored by:		NetApp, Inc.
Differential Revision:	https://reviews.freebsd.org/D43792