Remove dead code and fix a race condition in the node back end.
Race was between a store and a fetch, as follows:
1) Fetch: Search for the node, it is not found.
2) Store: Add the node to the database, remove the node from the negative cache.
3) Fetch: Add the node to the negative cache.
This would leave a node in the DB, cache, and negative cache.
Change visitLeaves to visit a snapshot so the ledger stays unlocked