feat: lfu wrapper for blockstore

[alexander-camuto]

• Implements an LFU eviction strategy nspired by the O(1) algorithm for LFU cache inviction http://dhruvbird.com/lfu.pdf.
• We maintain a linked list, where each node represents all the keys for blocks that are equally popular (popularity being measured as the number of reads).
• When the node reaches a maximum disk capacity in bytes it pops out keys to delete until it is below that capacity. The removed keys are the eldest in the store + the least popular (i.e the lowest level node in the frequency linked list). The evicted keys are then used to evict content from the blockstore.
• For O(1) in evicting content from this list we maintain a lookup table which maps CIDS/Hashes to pointer elements in the linked list.
• Tests for all operations
• LFU implements the BlockStore trait so is directly swappable into the PoP node as is.
• LFU automatically syncs keys from a blockstore loaded from disk -- so it can recover from reboots.

[alexander-camuto]

solves #14