Implement Drop for SnapshotService

[sgkim126]

Currently, the main thread will exit while the snapshot service is creating snapshots.

[remagpie]

I think we merged a similar implementation in #1869 (in snapshot branch).
Would you check it first?

[foriequal0]

There might be a race condition where the spawned thread exits first, so joins.remove() is called first and joins.insert() is called later.

How about cleaning it lazily?

fn new_blocks() {
	...
	let arc = Arc::new(());
	let weak = Arc::downgrade(&arc);
	let join = spawn(move || {
		let arc = arc; // If the stack dies, the arc dies. 
	    ...
	}
	let mut joins = joins.lock();
	joins.push((weak, join));
	joins.retain(|(weak, _)| Weak::upgrade(weak).is_some());
	...
}

[remagpie]

I think it can be solved by locking self.joins in the upper line.

let joins = Arc::clone(&self.joins);
let self_join = self.joins.lock();
let join = spawn(move || { ... });
self_join.insert(id, join);

The variable name is a bit awkward, but I think you'll get the point.

[sgkim126]

IMO, @remagpie's looks simple. @foriequal0 how about it?

[sgkim126]

I think we merged a similar implementation in #1869 (in snapshot branch).
Would you check it first?

Ah. Yes, it is fixed in the snapshot branch. I'll close it.