Use numbered subdirectories to avoid having too many files in one dir.
Add appropriate tests.
Also fix an issue where the mmap_open LRU cache could inappropriately
open a file twice because it was using the optional "newsize"
parameter as a key -- now lrucache can be given a slice object that
describes which arguments are important.
There's some bug with the testing harness where placing e.g.
from du import du
in nilmdb/utils/__init__.py doesn't quite work -- sometimes the
module "du" replaces the function "du". Not exactly sure why;
we work around that by just renaming files so they don't match
the imported names directly.