bfdd: simplify timer data structure

bfdd: simplify timer data structure

Remove some legacy left overs of the old timer data structure bits and
use a simpler version:

  We always keep the current configuration in the timer structure, but
  also keep the running timers (before poll transition) in
  `cur_timers`.

  With this we can remove `new_timers` and avoid timer copy
  configuration copy on final handler (this also simplifies peer
  show command).

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>