Removes BucketCounter
Adds test_stats to test_storage.py

This is used to test the new bucket_count reported by the server,
as well as the new stats total_leased_sharecount and
total_leased_used_space.
Fixes error in get_total_leased_sharecount_and_used_space

When share_count is 0, used_space will be None. Since the caller should
not have to worry about this, we check before returning the values.