Don't start watchdog timers unnecessarily

In particular, restart the timer with each incoming Diameter message,
only when the previous timer has expired. Doing so has been seen to
result in high lock contention at load, as in the example below:

(diameter@test)9> lcnt:conflicts([{print, [name, tries, ratio, time]}]).
                lock   #tries  collisions [%]  time [us]
               -----  ------- --------------- ----------
          bif_timers  7844528         99.4729 1394434884
              db_tab 17240988          1.7947    6286664
           timeofday  7358692          5.6729    1399624
           proc_link  4814938          2.2736     482985
        drv_ev_state  2324012          0.5951      98920
           run_queue 21768213          0.2091      63516
             pollset  1190174          1.7170      42499
            pix_lock     1956          2.5562      39770
            make_ref  4697067          0.3669      20211
           proc_msgq  9475944          0.0295       5200
         timer_wheel  5325966          0.0568       2654
           proc_main 10005332          2.8190       1079
     pollset_rm_list    59768          1.7752        480

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