Make ServerHash a String

Best not to ascribe any special meaning to it.

src/secrets.rs

@@ -48,8 +48,9 @@ impl From<String> for ServerPadlock {
     }
 }
 
+/// Not actually a hash, just a random value handed out to servers on request.
 #[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct ServerHash(pub Vec<u8>);
+pub struct ServerHash(pub String);
 
 #[derive(Deserialize)]
 pub struct PadlockGenerationSecret(pub SecretVec<u8>);