We want to be able to add the ipv4, udp, tcp, and arp stacks separately and do so from InternetStackHelper.

The idea is that InternetStackHelper would need to create only a single object for each element of the stack: UdpL4Protocol, TcpL4Protocol, Ipv4L3Protocol, and, ArpL3Protocol and each of these objects should be aggregated separately to a Node.

This means that:
1) we need to aggregate together multiple objects: TcpL4Protocol must aggregate itself with TcpSocketFactoryImpl. The same for UdpL4Protocol and UdpSocketFactoryImpl, etc.

2) we need to allow each object already-aggregated to a node to be notified whenever a new object is aggregated to the node: this is needed to allow the UdpL4Protocol object to find the node's Ipv4L3Protocol object to register as a layer 4 protocol. So, we need to add support in src/core/object.h|cc for a NotifyNewAggregate virtual method which is invoked by AggregateObject whenever a new object is aggregated to an existing aggregate.