Move touch_item() and delete_item() from discache.py to dis.py
Log exception traceback
Create class item for non-root nodes
Create class item for non-root nodes (for real this time)
Clean up node and rootnode classes
Add new nodes to items mapping from get_node() instead of node class constructor
Rename class rootnode to rootitem, change constructor arguments, and move functions
Move functions pop(), insert(), delete(), and touch() into item class
Remove empty directories during reclaim_files(), also code cleanups
Add helpers forget_node() and ready_to_lease()
Rewrite renew() using new helpers create_or_touch() and expire_files()