Reinforce the watchers's shields from cancellations

[nolar]

The shielding has been introduced in e0ae2ea for #147 for general stability of tests and for graceful termination of operators.

However, the shielding was only done for one operation in the finally: block — the scheduler closing — and that shield could itself be cancelled.

When the watcher was double-cancelled (with the second cancellation hitting somewhere in the finally: block), it left several tasks running in the background:

• The cancellation task itself (implicitly converted to a future/task by asyncio.shield()).
• The scheduler's internal job closing tasks.
• The scheduler's task for processing the job failures (implementation details).

At that time, the operators/tests were already going forward and not going to wait for these tasks to finish. This led to system resource (asyncio tasks) leakage in some tests; it was also probable in operators (though, they do not cancel the watchers often).

With this change, the anti-cancellation shield is reinforced to:

1. Shield the streams depletion (limited by an "exit timeout") too; previously, it was ignored and remained cancellable without getting to the scheduler closing.
2. Wait until the real completion of both the streams depletion and the scheduler closure; previously, the wait could be cancelled.

This ensures that under no circumstances any worker is left unattended after the watcher exits — all workers are now guaranteed to finish or be cancelled as part of the watcher's cancellation/finalisation.