Add B+Tree Node and Leaf size fine tuning options.

[koculu]

Available B+Tree options:

    /// <summary>
    /// Controls lock granularity of in-memory B+Tree that represents the mutable segment.
    /// </summary>
    public BTreeLockMode BTreeLockMode { get; set; } = BTreeLockMode.NodeLevelMonitor;

    /// <summary>
    /// The B+Tree node size.
    /// </summary>
    public int BTreeNodeSize { get; set; } = 128;

    /// <summary>
    /// The B+Tree leaf size.
    /// </summary>
    public int BTreeLeafSize { get; set; } = 128;