bgpd: optimize holdtime timer cancelling

bgpd: optimize holdtime timer cancelling

* bgp_packet.c: (bgp_update_receive) for every update received we queue
  an event just to cancel the holdtime timer, done in bgp_fsm_update().
  Instead cancel the timer directly an avoid a scheduling pass.

  This incidently fixes another problem found on a slow box, where thousands
of events threads were queued, and run, but never freed, because they are
moved to the unused list that grows without bounds.

Signed-off-by: Jorge Boncompte [DTI2] <jorge@dti2.net>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>