Experiment using tbb::aggregator for list management.

It's an improvement over full locking, but not as good as the really lazy aggregation without locking we tried previously. Additionally,
the aggregator is a preview feature in tbb, and only appears in version 4.0 anyway.
Revert "Experiment using tbb::aggregator for list management."

This reverts commit 069a8778cbba1e955ffc2f8e195b5f9000ca3cc1.
Added todo.
Added a bit of instrumentation.

To get a sense of how the lazy queue processing works in practice. It seems it's quite common to have nothing in the queue after processing, but occasionally we have _a lot_ in the queue.