[Bug] Zenoh continues scouting after session closed

[OlivierHecart]

Describe the bug

After a session is closed, some tasks are left running that periodically send scout messages and reply to scout messages.

To reproduce

1. Write a program that opens a Zenoh session, closes the Zenoh session and keeps running:

use zenoh::prelude::r#async::*;
#[async_std::main]
async fn main() {
    env_logger::init();
    let session = zenoh::open(Config::default()).res().await.unwrap();
    session.close().res().await.unwrap();
    futures::future::pending::<()>().await;
}

2. Run the program with orchestrator trace logs enabled:

$ RUST_LOG=zenoh::net::runtime::orchestrator=trace z_open_close

System info

Zenoh v0.7.0-rc (7728f0d)

[OlivierHecart]

This is a regression introduced by: 7db53e8