Fix authority-discovery cannot exit problem

[kigawas]

Related #3452

[parity-cla-bot]

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[parity-cla-bot]

It looks like @kigawas signed our Contributor License Agreement. 👍

Many thanks,

Parity Technologies CLA Bot

[kigawas]

BTW: there's some error message like:

2019-09-10 18:12:14 Failed to put hash 'Key(b"\x12 F \x87\x98\xc8\x0b\xe6S\x1cjdT1-\xb7\xd1PYb7k<\xca\x16\xcd\x18\xf9\x10\xd6\xfe\xa8^")' on Dht.
2019-09-10 18:12:14 Value for hash 'Key(b"\x12 \x94\xb4\x12\xff\xd7\x93J\xeay\xad\xf6\xdd\xb5\x19\x887\xaa\x9b&\xb3:T\xc1\xd7\xa4T\xc67\xd6\xdd\x81\xeb")' not found on Dht.

Not sure whether it's a feature

[bkchr]

Ty, but already fixed by: #3581

[kigawas]

np