Follow-up of cosmos/cosmos-sdk#2131.

After discussion on Slack, a suggestion to implement a fix:

1. For each new node in IAVL tree, assign an sequential integer ID. This ID never changes for the node.
2. Use this integer ID as "key" in the <key, value> pair stored in LevelDB.
3. The value part stored in LevelDB contains the node hash (existing LevelDB key).
4. All parent/child references must be through integer IDs.
5. A special "root" key stored in LevelDB has an value that contains the integer ID of the root node.