Use random salt for each fingerprint key

Salt length depended on fingerprint name length, which lead to weak salts in some cases
It's now a fixed length (12 bytes)

Akari.Prototype.Server/Services/IAuthManager.cs

@@ -11,6 +11,8 @@ namespace Akari.Prototype.Server.Services
     {
         IEnumerable<KeyValuePair<string, TimedEntry<AesGcm>>> Pairs { get; }
 
+        void GenerateSalt(string name);
+
         void Auth(string name, byte[] token);
 
         bool Remove(string name);