Fixed three bugs:

  1) Peter Bigot @ Rincon noted that the changed flag was incorrectly cleared
     and set, such that elapsed time would not be considered in trickle timers.
     This caused trickle timers to fire late.

  2) The possibility of a subtraction wraparound could cause timers to
     have very large values and fire very late.

  3) A repetition of subtracting the elapsed time (in both the fired
     event and in adjustTime) would cause timers to fire early.