Destroy reference to taskmgr after all tasks are done

Destroy reference to taskmgr after all tasks are done

We were clearing the pointer to taskmgr as soon as isc_taskmgr_destroy()
would be called and before all tasks were finished.  Unfortunately, some
tasks would use global named_g_taskmgr objects from inside the events
and this would cause either a data race or NULL pointer dereference.

This commit fixes the data race by moving the destruction of the
referenced pointer to the time after all tasks are finished.