Make sure guard is not considered unused

[mystenmark]

No description provided.

[andll]

@mystenmark I think this part of code is very suspicious.
In Ok() branch we spawn the task and run state.handle_messages, in Err() branch we spawn the task and take the guard and then drop it. Does it make more sense to just drop this logic entirely and do something like

let mut receiver = state.receiver.clone().try_lock_owned().expect("Duplicate call to NodeSyncState::start() "); // 

[mystenmark]

We could. But I was very scared of putting an unwrap() into the validator that is never called. It's much scarier than putting in an unwrap() that is called and you know is safe. I don't know how to prove that nothing in the code will ever make a duplicate call to the start function.

[andll]

Okay, but it is still strange piece of code :/