null versionId objects

[0x180]

The implementation handles put/get/delete operations for null versionId objects. If versionId attribute isn't set for an object, it's treated as null.

It ensures that there is always exactly one object version with null versionId in a bucket at any given time.

The file name for the object with a null versionId is null in the versioning directory.

The listing of object versions follows this strategy:

1. Locate the object with the null versionId first.
2. Begin listing the other object versions.
3. Insert the null versionId object into its correct position in the list by comparing modification dates to maintain the proper order.

Fixes #864