syn2mas: panic in writer tasks can lead to stalling

[reivilibre]

If a syn2mas MAS writer task panics, it doesn't shut down properly, but depending on the exact failure:

• if all tasks panic, I suspect it will generally get stuck waiting for them to be ready to accept new tasks
• if not all of them panic, it will be detected that some panicked — at the end

Originally posted by @reivilibre in #3878 (comment)

[sandhose]

One neat way to solve this is to use a tokio_util::sync::CancellationToken with a DropGuard.

If you put that guard in each task, when the task crashes, it cancels the token, and in other places you can regularly checked if the token ::is_cancelled(). This is useful as well to implement graceful shutdowns on SIGINTs & co.