dht: use SHA265 instead of SHA512 for the op token

The op token is renewed regularly (around every 30 minutes),
so SHA256 provides enough security for this non-sensitive token.
SHA256 takes 32 bytes instead of 64 bytes for SHA512,
and the token is included in every listen requests,
renewed every 30 seconds, and every answer to get.
In the context of limited bandwith usage, using SHA256 seems more appropriate.
This change is backward and forward compatible.