I can't remember zombies still being near me if I logged out and back in in single player and I would assume it works the same on the server. Everything *should* despawn and basically reset, I would think. Obviously, if you're in a POI and are near where zombies spawn, you might get zombies that you haven't killed already spawning right next to you. And if it's before 4am on horde night, you're going to very quickly have zombies attacking again but they should start spawning at a distance and not appear right next to you even if they were there when you restart the server. But that's entirely a guess based on single player.