network/src/legacy/gossip: Wrap GossipEngine in Arc Mutex & lock it on use

`GossipEngine` in itself has no need to be Send and Sync, given that it
does not rely on separately spawned background tasks anymore.
`RegisteredMessageValidator` needs to be `Send` and `Sync` due to the
inherited trait bounds from implementing `GossipService`. In addition
`RegisteredMessageValidator` derives `Clone`. Thereby `GossipEngine`
needs to be wrapped in an `Arc` and `Mutex` to keep the status quo.