Improve code quality

[JakkuSakura]

Hi, thank's for the effort being done.

Recently, I hit an issue that if scheduler.spawn() is called in the code, without actual tasks, my server blocks.

I started reviewing code in this crate. I noticed the following points to improve

1. use of Arc<Mutex<bool>> can be written as Arc<AtomicBool>
2. use of

   tokio::spawn(async move {
   if let Err(e) = tx.send(uuid) {
       error!("Error sending deletion {:?}", e);
   }
   });

   can be replaced with just tx.send()

3. use of

   let mut start_rx: Option<Receiver<bool>> = None;
   std::mem::swap(&mut start_rx, &mut *w);
   start_rx

   can be replaced as w.take()

[haydenflinner]

Interestingly, the ::init function is not mentioned in the example of how to use the scheduler, it gets called implicitly when adding a job. But we don't use atomic swap for writing to inited, just acquire a lock and then write true, so it seems that if someone had a few clones of the JobScheduler (uninitted) and called add on them at the same time, the actors may get started twice. Seems to be likely harmless, though.