client/network-gossip: Move sink IO outside of state_machine

`ConsensusGossip` is supposed to be a deterministic state machine.
`GossipEngine` wrapping `ConsensusGossip` should handle IO operations.

This commit moves the `message_sink` IO operations to `GossipEngine`.
More specifically on incoming messages a `GossipEngine` calls
`ConsensusGossip::on_incoming` to validate and register the messages.
`ConsensusGossip` returns the valid messages which are then forwarded by
`GossipEngine` to the upper layer via the `message_sinks`.