Rework storage for Accessors

The initial storage (trees/nodes) and lifecycle model for Accessors left
open a race between node self-cleanup (removing the associated entry
from a tree on drop) and racing node re-parenting (as part of node
adoption or orphaning).

This changes how nodes are cleaned up from their containing tree, and
simplifies indexing of nodes in said tree to use (opaque) pointers,
rather than arbitrary numeric IDs.

Fixes #675

Author: pfmooney