bring up the rest of the rename code:

  - add a new index operation: index_droppings_rename(src,dst): we call
    this when we rename a container.  the index may need to know this if
    it stores its data outside of the container directory (e.g. externally
    from the container).

  - XXX: note the error recovery in rename() is not complete.  to rename
    a container, we have to rename a bunch of files in the back end IOStore
    and we also have to copies files (e.g. METADIR files) from the old
    canonical location to the new one, update metalinks, etc.   in an
    ideal world if we have a failure we'd like to be able to roll it
    all back (e.g. kind of like aborting a transaction), but we don't
    keep enough info to do that.   (this was an issue before
    ContainerIndex abstraction, and it is beyond the scope of ContainerIndex
    to fix it).