kad: Store peers in btreemap for kbuckets

[lexnv]

At the moment, we store the peers of a k-bucket in a vector.

Investigate if storing peers in a BTreeMap in ascending order of their distance from local key would bring any benefit performance-wise. Considering that we need to sort the records on closest_iter depending on the target key, I would lean towards not having to pay for the performance hit of BTreeMap, would be good to have a second look.

litep2p/src/protocol/libp2p/kademlia/bucket.rs

Lines 58 to 62 in e0b7118

	pub struct KBucket {
	// TODO: store peers in a btreemap with increasing distance from local key?
	nodes: Vec<KademliaPeer>,
	}