Bug fix: Stop the DAO lifetime timer in RPL instances when released.

Clearing the memory for an active etimer/ctimer might corrupt the
timer list and cause other timers to be lost or infinite loops. The
DAO lifetime timer is only used when RPL route lifetime is not
infinite but then the timer will cause problems if not stopped.
Merge pull request #882 from nfi/dao-lifetime-timer

Stop the DAO lifetime timer in RPL instances when released.