meaningless cache adding in the pruning

[cool-develope]

Context

Currently, iavl v1 nodeCache is not working efficiently. It adds the node to the cache whenever it gets the node. There are some meaningless usecases. Furthermore, it is leading to extra delays.

• Pruning: Those nodes are added to the cache before deleting.
• Querying: The querying is random access, there is no guarantee of re-visiting.

Proposal

• Only adds to the cache whenever the node is created. This way, we can leverage access to the top-level nodes of the tree like the root, actually these nodes are highly frequently visited.
• Use sync.Pool in the GetNode to leverage the memory re-use which is introduced in iavl v2.