Enable filtering down nodes in scylla::transport::load_balancing::DefaultPolicy

[havaker]

Our node up/down markers do not work reliably yet (#607), but are required for a true is_alive check:

scylla-rust-driver/scylla/src/transport/load_balancing/default.rs

Lines 350 to 354 in adb18f7

	fn is_alive(node: &NodeRef<'_>) -> bool {
	// For now, we leave this as stub, until we have time to improve node events.
	// node.is_enabled() && !node.is_down()
	node.is_enabled()
	}