task: do not allow queue loops in priority queueing mode

Tasks using non-priority FIFO dependency tracking can insert themselves
into their own queue, to run the Task again immediately after it exits.

For priority queues, this attempts to splice the post-exec queue into
itself, which doesn't seem like a good idea.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/task.cc

@@ -328,6 +328,7 @@ namespace crucible {
 	TaskState::insert(const TaskStatePtr &task)
 	{
 		THROW_CHECK0(invalid_argument, task);
+		THROW_CHECK2(invalid_argument, m_id, task->m_id, m_id != task->m_id);
 		PairLock lock(m_mutex, task->m_mutex);
 		if (!task->m_run_now) {
 			task->m_run_now = true;