make polkadot die graciously (#2056)

While investigating some db migrations that make the node startup fail,
I noticed that the node wasn't exiting and that the log file were
growing exponentially, until my whole system was freezing and that makes
it really hard to actually find why it was failing in the first place.

E.g:
```
 ls -lh /tmp/zombie-01a04c2a2c0265d85f6440cf01c0f44a_-51319-uyggzuD4wEpV/bob.log
 32,6G oct 27 11:16 /tmp/zombie-01a04c2a2c0265d85f6440cf01c0f44a_-51319-uyggzuD4wEpV/bob.log
```

This was happening because the following errors were being printed
continously without the subsystem main loop exiting:

From dispute-coordinator:
```
WARN tokio-runtime-worker parachain::dispute-coordinator: error=Subsystem(Generated(Context("Signal channel is terminated and empty.")))
```

From availability recovery:
```
Erasure task channel closed. Node shutting down ?
```

Signed-off-by: Alexandru Gheorghe <[email protected]>