Consider using multiple iptables chains for firewalling on the router

[ldruschk]

Currently, we have pretty complex iptables rules on the router that require some iptables rules to be in the correct order. We should evaluate whether it makes sense to have multiple chains for our firewall rules. The we just need to ensure that we jump into the different tables in the correct order, but can add rules to the different tables independently without worrying about whether they must be before or after the rules in other chains.

The chains should be structured in such a way that the order of the rules within one chain does not matter but only the order of the chains matters.