Use appropriate timeout_t* instead of void* in tcp_timer_activate()

Suggested by:               imp
Differential Revision:      https://reviews.freebsd.org/D2154
Reviewed by:                imp, jhb
Approved by:                jhb

Provide better debugging information in tcp_timer_activate() and
tcp_timer_active()

Differential Revision:      https://reviews.freebsd.org/D2179
Suggested by:               bz
Reviewed by:                jhb
Approved by:                jhb

Fix an old and well-documented use-after-free race condition in
TCP timers:
 - Add a reference from tcpcb to its inpcb
 - Defer tcpcb deletion until TCP timers have finished

Differential Revision:      https://reviews.freebsd.org/D2079
Submitted by:               jch, Marc De La Gueronniere <mdelagueronniere@verisign.com>
Reviewed by:                imp, rrs, adrian, jhb, bz
Approved by:                jhb
Sponsored by:               Verisign, Inc.