Refactoring of the time-step communication tasks

Significant re-factoring of the way the time-step sizes are being exchanged.

Same as !1455 (closed) but without the last batch of changes.

Summary:

• A new top-level task collects the time-step sizes from the super level to the top-level. This was formerly done by making engine_collect_end_of_step() recurse.
• The timestep, timestep_limiter, and timestep_sync tasks all unlock that top-level task.
• engine_collect_end_of_step() now only loops (via threadpool) over the local top-level cells. No recursion any more.
• For each pair of top-level cells in the proxies we construct a pair of send/recv comm tasks.
• That comm task packs up the dt of the whole hierarchy sends it and unpacks the time-step sizes.
• The top-level time-step collection task unlocks the send.
• The individual per-species tend communication tasks that used to live at the super level are removed.
• The second call to engine_launch() done every step to deal with the timestep limiter effect is removed (as it is now properly dealt with by the top-level task dependency)

This should help speed up the smallest steps by reducing the level of the plateau we usually see in the "main sequence" plots.