state sync: randomize order in which parts are requested

[saketh-are]

In the current state sync implementation, every node in the network will attempt to obtain state parts in roughly the same order (modulo any variation due to parallelism of buffer_unordered):

nearcore/chain/client/src/sync/state/shard.rs

Lines 85 to 98 in 4e87bae

	tokio_stream::iter(0..num_parts)
	.map(|part_id| {
	let future = downloader.ensure_shard_part_downloaded(
	shard_id,
	sync_hash,
	part_id,
	header.clone(),
	cancel.clone(),
	);
	respawn_for_parallelism(&*future_spawner, "state sync download part", future)
	})
	.buffer_unordered(MAX_PARALLELISM_PER_SHARD_FOR_FAIRNESS)
	.try_collect::<Vec<_>>()
	.await?;

The peer selection mechanism is designed such that requests for the same state parts will be routed to the same hosts. The motivation is that we can benefit from caching of parts on the host side.

We should shuffle the order of requests on the client side to avoid generating spikes of requests to the same hosts.