Believe that pruned_blocks are no longer needed (#5605), will tackle this in another PR

One downside of this approach is that we never remove anything manually and just rely on the LRU to remove items. However, that means that over time we will have the maximum amount of items in every LRU for every subscription. But in reality probably does not matter, for these 512 items thats approx. 16kb of memory usage then 🤷

I think it's quite "strange/hard to understand" use a LRU for the finalized blocks when it's treated as a FIFO queue.

Personally I would just use a fixed size ring buffer but probably less efficient to do lookups.

An alternative is create a wrapper type such as:

/// A wrapper type to utilize a LruMap as FIFO queue.
struct MostRecentFinalizedBlocks(LruMap<Block::Hash, ()>);

impl FinalizedBlocksFifo {
    fn insert(&mut self, block: Block::Hash) {
        self.finalized.insert(block, ());
     }

    fn contains(&mut self, block: &Block::Hash) -> bool {
        self.0.peek(block).is_some()
    }
}