task: flatten queues of dependent Tasks

Suppose Task A, B, and C are created in that order, and currently running.
Task T acquires Exclusion E.  Task B, A, and C attempt to acquire the
same Exclusion, in that order, but fail because Task T holds it.

The result is Task T with a post-exec queue:

        T, [ B, A, C ]  sort_requested

Now suppose Task U acquires Exclusion F, then Task T attempts to acquire
Exclusion F.  Task T fails to acquire F, so T is inserted into U's
post-exec queue.  The result at the end of the execution of T is a tree:

        U, [ T ]  sort_requested
             \-> [ B, A, C ] sort_requested

Task T exits after failing to acquire a lock.  When T exits, T will
sort its post-exec queue and submit the post-exec queue for execution
immediately:

        Worker 1: U, [ T ]  sort_requested
        Worker 2: A, B, C

This isn't ideal because T, A, B, and C all depend on at least one
common Exclusion, so they are likely to immediately conflict with T
when U exits and T runs again.

Ideally, A, B, and C would at least remain in a common queue with T,
and ideally that queue is sorted.

Instead of inserting T into U's post-exec queue, insert T and all
of T's post-exec queue, which creates a single flattened Task list:

        U, [ T, B, A, C ]   sort_requested

Then when U exits, it will sort [ T, B, A, C ] into [ A, B, C, T ],
and run all of the queued Tasks in age priority order:

        U exited, [ T, B, A, C ]   sort_requested

        U exited, [ A, B, C, T ]

        [ A, B, C, T ] on TaskConsumer queue

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

lib/task.cc

@@ -331,8 +331,11 @@ namespace crucible {
 		PairLock lock(m_mutex, task->m_mutex);
 		if (!task->m_run_now) {
 			task->m_run_now = true;
+			// Move the task and its post-exec queue to follow this task,
+			// and request a sort of the flattened list.
 			m_sort_queue = true;
-			append_nolock(task);
+			m_post_exec_queue.push_back(task);
+			m_post_exec_queue.splice(m_post_exec_queue.end(), task->m_post_exec_queue);
 		}
 	}