Investigate better lifecycle of SimulatedGossip

The lifecycle of `SimulatedGossip` is controlled by `SimulatedNetwork`. This feels unnatural as the `Gossip` functionality is usually part of the node. It is also not entirely clear what happens with the `SimulatedGossip` wires when a node is restarted (i.e., destroyed and recreated).

We should investigate if this can be changed. `SimulatedGossip` should be created and destroyed with its node, and it should be registered and unregistered with `SimulatedNetwork`.

Start by tracing the lifecycle of SimulatedGossip through SimulatedNetwork, including what happens when a node is destroyed and recreated. Done means determining whether SimulatedGossip can be created and destroyed with its node while being registered and unregistered with SimulatedNetwork, with restart behavior understood.

Written by the indexing model from the issue text.