MINOR: Refactor snapshottableSize method for better readability and simplicity

[chanbinme]

Motivation

The snapshottableSize(long epoch) method previously contained unnecessary nested conditions and multiple redundant return baseSize(); statements. This refactor aims to simplify the method's control flow and improve code readability.

Changes

• Changed the condition from epoch == LATEST_EPOCH to epoch != LATEST_EPOCH to avoid unnecessary else block usage.
• Consolidated the redundant return baseSize(); statements into a single return at the end of the method.
• Ensured the iterator logic executes only when necessary, resulting in clearer control flow.

Before

int snapshottableSize(long epoch) {
    if (epoch == LATEST_EPOCH) {
        return baseSize();
    } else {
        Iterator<Snapshot> iterator = snapshotRegistry.iterator(epoch);
        while(iterator.hasNext()) {
            Snapshot snapshot = iterator.next();
            HashTier<T> tier = snapshot.getDelta(SnapshottableHashTable.this);
            if (tier != null) {
                return tier.size;
            }
        }
        return baseSize();
    }
}

After

int snapshottableSize(long epoch) {
    if (epoch != LATEST_EPOCH) {
        Iterator<Snapshot> iterator = snapshotRegistry.iterator(epoch);
        while(iterator.hasNext()) {
            Snapshot snapshot = iterator.next();
            HashTier<T> tier = snapshot.getDelta(SnapshottableHashTable.this);
            if (tier != null) {
                return tier.size;
            }
        }
    }
    return baseSize();
}

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Please let me know if any further adjustments are needed.
Thank you for your review!