fs-cache: Add LRU Struct

[pushkarm029]

No description provided.

[pushkarm029]

To remove few expensive clones, we can consider this later:

pub struct CacheEntry<V> {
    value: V,
    size: usize,
    top: Option<usize>,
    bottom: Option<usize>,
    id: usize,
}

pub struct LRUCache<K, V> {
    entries: HashMap<K, CacheEntry<V>>,
    keys: HashMap<usize, K>,
    head: Option<usize>,
    tail: Option<usize>,
    memory_used: usize,
    max_memory: usize,
    id_counter: usize,
}