Try to keep documented promise that

	The heuristic 'auto' should behave as good as ZBlk0 in case of wide changes

Previously the heuristic code would use ZBlk0 only in append case after
switching previously filled-up block to ZBlk0, and the case of arbitrary
changes were unconditionally using ZBlk1.

Now we see if the append is small and use ZBlk1 only then. If the append
is big, or if the change is itself is big - we use ZBlk0.

This should restore documented behaviour that heuristic behaves as good
as ZBlk0 in case of wide changes.

TODO update benchmark/test to cover this case.