Implement nth method for scylla::transport::locator::ReplicaSetIterator more efficiently

[havaker]

Manual implementation for ChainedNTS variant would be more efficient than just skipping nodes via .next() (#612 (comment)).

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

Lines 535 to 550 in adb18f7

	fn nth(&mut self, n: usize) -> Option<Self::Item> {
	match &mut self.inner {
	ReplicaSetIteratorInner::Plain { replicas: _, idx } => {
	*idx += n;
	self.next()
	}
	_ => {
	for _i in 0..n {
	self.next()?;
	}
	self.next()
	}
	}
	}