Rework consensus instance communication with the network worker (#958)
Up to now consensus instances used the main channel to communicate with the background network worker. This lead to a race condition when sending a local collation and dropping the router before driving the send local collation future until it is finished. This pr changes the communication between worker and the instances to use their own channels. This has the advantage that we don't need an extra `DropConsensusNetworking` message as the network is dropped automatically when the last sender is dropped.