task: add an insert method for priority-queueing Tasks by age

Task started out as a self-organizing parallel-make algorithm, but ended
up becoming a half-broken wait-die algorithm.  When a contended object
is already locked, Tasks enter a FIFO queue to restart and acquire the
lock.  This is the "die" part of wait-die (all locks on an Exclusion are
non-blocking, so no Task ever does "wait").  The lock queue is FIFO wrt
_lock acquisition order_, not _Task age_ as required by the wait-die
algorithm.

Make it a 25%-broken wait-die algorithm by sorting the Tasks on lock
queues in order of Task ID, i.e. oldest-first, or FIFO wrt Task age.
This ensures the oldest Task waiting for an object is the one to get
it when it becomes available, as expected from the wait-die algorithm.

This should reduce the amount of time Tasks spend on the execution queue,
and reduce memory usage by avoiding the accumulation of Tasks that cannot
make forward progress.

Note that turning `TaskQueue` into an ordered container would have
undesirable side-effects:

 * `std::list` has some useful properties wrt stability of object
 location and cost of splicing.  Other containers may not have these,
 and `std::list` does have a `sort` method.

 * Some Task objects are created at the beginning and reused continually,
 but we really do want those Tasks to be executed in FIFO order wrt
 submission, not Task ID.  We can exclude these tasks by only doing the
 sorting when a Task is queued for an Exclusin object.

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

include/crucible/task.h

@@ -47,6 +47,10 @@ namespace crucible {
 		/// been destroyed.
 		void append(const Task &task) const;
 
+		/// Schedule Task to run after this Task has run or
+		/// been destroyed, in Task ID order.
+		void insert(const Task &task) const;
+
 		/// Describe Task as text.
 		string title() const;