This commit introduces auto-tuning of objsize_hint based on a rough
metric of the average object size when the storage is between 25% and
75% full. A change in objsize_hint triggers resizing of the
pre-allocated log-regions, which is no issue when they shrink (because
of increased objsize_hint).

But increasing the log-regions requires relevant contigious storage
regions, for which LRU might need to remove a lot of objects.

So I am not sure yet if this is a good idea. We could maybe merge the
feature, but leave it disabled for now...