Penciller back-pressure - Phase 1

There were issues with how the Penciller behaves under ehavy write
pressure - most particularly where there are a large number of keys per
update (i.e. 2i heavy objects).   Most immediately the attempt to chekc
whether the l0 file was ready slowed down the process of producing the
L0 file - so back-pressure created more back-pressure.

Going forward want to alter this most significantly as also the work
queue can build up unsustainably. there needs to be some pausing
prompted by the bookie on 'returned', and the use of 'returend when the
work queue exceeds a threshold.

src/leveled_log.erl

@@ -93,6 +93,8 @@
         {info, "Rename of manifest from ~s ~w to ~s ~w"}},
     {"P0028",
         {info, "Adding cleared file ~s to deletion list"}},
+    {"P0029",
+        {info, "L0 completion confirmed and will transition to not pending"}},
     
     {"PC001",
         {info, "Penciller's clerk ~w started with owner ~w"}},