The old approach of getting state tries involved instantiating a
new state database and using it to instantiate a trie. What we didn't
realize was that state database needed to have Close() called on it,
which we didn't offer a way for plugins to do, resulting in memory
leaking.
This approach reuses the primary trie database associated with the
blockchain object, albeit a bit indirectly. This will allow access
to recent tries that are stored in memory, where previously only
tries that had been committed to disk were accessible.
