Reduce EnvRef:set_node() time tenfold by postponing the dayNightDiff update until it is actually needed

Don't set m_removed on dead players (dead players are indicated by hp == 0). Local
damage flash is shown whatever the cause was (even from Lua set_hp). PlayerCAO
damage flash matches duration of local damage flash. Fall damage is dealt much more consistently (this is done by disallowing jumping when speed.Y is very negative, up to now jumping could sometimes negate fall damage)