chore(net): get and peek for LruCache

[emhane]

Adds methods get and peek to LruCache, which are needed for #6559

[mattsse]

this is the same as:

Suggested change

	let _ = self.inner.get(entry)?;
	self.iter().next()
	let _ = self.inner.get(entry)?;
	Some(entry)

?

[emhane]

not if the types have a custom PartialEq impl, consider type

struct MyType {
    id: i32,
    colour: i32,
}

impl PartialEq for MyType {
    fn eq(&self, other: &Self) -> bool {
        self.id == other.id
    }
}

impl Hash ..

then we can have two instances with different colour, that will both have the same key

let green = MyType { id: 1, colour: 32 };
let blue = MyType { id: 1, colour: 34 };

let mut cache = LruCache::new();
cache.insert(green);
let _ = cache.get(&blue); // this will exist, but it will actually be green that is in the map

this extension to LruCache just makes it safe to use in the codebase kind of, cause we have a couple of types that impl custom PartialEq on a subset of fields. using a HashMap for these types makes way more sense though.