the root private key should not be used for sealing Biscuit tokens

[Geal]

I'm not sure if it will result in a big vulnerability, but it would be better to have a dedicated secret key for this: https://github.com/PierreZ/record-store/blob/master/record-store/src/main/java/fr/pierrezemb/recordstore/auth/BiscuitManager.java#L80

[PierreZ]

Thanks @Geal for the issue!

The BiscuitManager trully need some improvements indeed, I'll add another private key for this 😄