Function decay_importance 

pub fn decay_importance(
    conn: &Connection,
    days: u64,
    factor: f64,
    floor: f64,
) -> Result<u64>

Gradually decay importance for nodes not accessed in days.

Reduces importance by factor (e.g. 0.9 = 10% decay). Nodes with importance at or below floor are not decayed further. Nodes with the pinned tag are protected from decay.

Returns the number of nodes decayed.