HashSet sometimes uses compareTo() instead of equals() and this breaks the comparison of net.minecraft.server.NextTickListEntry (the only place where HashTreeSet is used).
In this cases duplicate entries could be added to the HashSet of HashTreeSet, because NextTickListEntry.compareTo() does not return 0, even if NextTickListEntry.equals() returns true.
ObjectOpenHashSet never uses compareTo(), so the inconsistencies of NextTickListEntry cause no problems.
Fixes https://github.com/PaperMC/Paper/issues/588
