Avoid looking up previous version of nodes that are new

[afterdusk]

Overview

When writing tree nodes to storage, we look up previous versions of the node to create the TreeNodeWithPreviousValue struct. However, this is not necessary when we know a node is new and does not have a previous value. By keeping track of whether a node is new, we save on incurring an unnecessary round trip to the database.

This PR performs the requisite change.

I would have liked to add a test for this change, however I cannot think of a straightforward way to assert that the database is not read beyond the initial preload. I believe this is something that can be addressed by bringing in a mocking library to mock out the Database trait, or implementing an in-memory database which we can mock. Filed an issue regarding this: #332

Manual Test

Added the following test to append_only_zks.rs, which batch inserts 10 nodes on an empty tree and prints database access metrics:

#[tokio::test]
async fn test_get_count() -> Result<(), AkdError> {
    let num_nodes = 10;
    let node_set = gen_nodes(num_nodes);

    let database = AsyncInMemoryDatabase::new();
    let db = StorageManager::new(database.clone(), None, None, None);
    let mut azks = Azks::new(&db).await?;

    db.begin_transaction();
    azks.batch_insert_nodes(&db, node_set, InsertMode::Directory)
        .await?;
    db.log_metrics(log::Level::Trace).await;

    Ok(())
}

The test was ran with the following command:

$ cargo test test_get_count -- --nocapture

If the change works, no gets are expected as the insert was performed on an empty tree with nothing to preload.

Before

On the main branch, with 7878bcb cherry-picked:

===================================================
============ Database operation counts ============
===================================================
    SET 1, 
    BATCH SET 0, 
    GET 19, 
    BATCH GET 0
    TOMBSTONE 0
    GET USER STATE 0
    GET USER DATA 0
    GET USER STATE VERSIONS 0

19 gets are performed.

After

On this PR's branch:

===================================================
============ Database operation counts ============
===================================================
    SET 1, 
    BATCH SET 0, 
    GET 0, 
    BATCH GET 0
    TOMBSTONE 0
    GET USER STATE 0
    GET USER DATA 0
    GET USER STATE VERSIONS 0

Indeed the number of gets are now 0.

[eozturk1]

Can we get the statistics on an integration test suite to see whether we are using any GET calls that hits the database? Example in #223.

[slawlor]

We'll follow up with automated tests once we have a storage mock imported

[eozturk1]

Thanks for sharing the metrics, looks great!