mongo::PeriodicTask::Runner cleanup

I'm testing your application using c++ mongoclient with valgrind and found, that the PeriodicTasks stored in PeriodicTask::Runner aren't destroyed during application shutdown. Off course I know the OS will do that for us, but just to make the code cleaner.

Possible fix:
PeriodicTask::Runner::~Runner() {
scoped_spinlock lk( _lock );
for ( size_t i=0; i<_tasks.size(); i++ )

}