Bucket directory tree is 2 levels hashed deeper.

Example, "default" bucket would go into the
BUCKET_DIR/00/2f/default-bucket subdirectory.

cbugg: close bug-388
NodeAllocs aren't easy to test.

Treaps have randomness, so need to be able to control that before we
should expect repeatable tests on node allocations.  Until then,
ignore NodeAllocs in stats validation tests.

cbugg: close bug-462