Optimize `PrecomputeReplicas` for the case when `rack count <= 1`

[havaker]

Memory footprint of precomputing replica sets for NTS strategies can be reduced when datacenters have at most one rack.

NTS dc-filtered replica lists computed for rf = n are prefixes of replica lists computed for rf = n+1 (if rack_count <= 1), so they can be "compressed" - only the largest list can be computed.

scylla-rust-driver/scylla/src/transport/locator/precomputed_replicas.rs

Lines 130 to 159 in adb18f7

	let rack_count = dc_rep_data.get_rack_count();
	let compressed_replica_ring_rf = repfactors.range(..=rack_count).next_back();
	let replica_ring_rf_above_rack_count = repfactors.range((rack_count + 1)..);
	let produce_replica_ring_iter = |rf| {
	let ring_iter = dc_rep_data.get_dc_ring().iter().map(|(token, _)| {
	let cur_replicas: Replicas = replication_data
	.nts_replicas_in_datacenter(*token, dc_name, rf)
	.cloned()
	.collect();
	(*token, cur_replicas)
	});
	TokenRing::new(ring_iter)
	};
	let compressed_replica_ring =
	compressed_replica_ring_rf.map(|rf| PrecomputedReplicasRing {
	replicas_for_token: produce_replica_ring_iter(*rf),
	max_rep_factor: *rf,
	});
	let above_rack_count_replica_rings = replica_ring_rf_above_rack_count
	.map(|rf| (*rf, produce_replica_ring_iter(*rf)))
	.collect();
	let dc_precomputed_replicas = DatacenterPrecomputedReplicas {
	compressed_replica_ring,
	above_rack_count_replica_rings,
	};