When isc_task_purgeevent() is called for and 'event', the event, in
the meanwhile, could in theory get processed, unlinked, and freed.
So when the function then operates on the 'event', it causes a
segmentation fault.
The only place where isc_task_purgeevent() is called is from
timer_purge().
In order to resolve the data race, call isc_task_purgeevent() inside
the 'timer->lock' locked block, so that timerevent_destroy() won't
be able to destroy the event if it was processed in the meanwhile,
before isc_task_purgeevent() had a chance to purge it.
In order to be able to do that, move the responsibility of calling
isc_event_free() (upon a successful purge) out from the
isc_task_purgeevent() function to its caller instead, so that it can
be called outside of the timer->lock locked block.
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