Prior to this commit, when using discv4 (Kademlia) to find peers, there is a
crash after a few minutes. It occurs for most of us on Eth1 mainnet, and
everyone on Ropsten.
Tested for about 4 days constant operation on Ropsten. The crash which occured
every few minutes no longer occurs, and discv4 keeps working.
Signed-off-by: Jamie Lokier <jamie@shareable.org>