Moved HMAC functionality from digest module to CrtAuthCodec.

* Since message verification is now part of the deserializeAuthenticated
  interface in CrtAuthCodec it seems reasonable move it.
* Removed the DigestAlgorithm abstraction and the unused SHA-1
  implementation from the digest module.
* Moved the HMAC variant from VerifiableDigestAlgorithm to a static method
  in CrtAuthCodec.
* Removed the somewhat confusing double builder parameter to CrtAuthServer
  where both secret and DigestAlgorithm filled the same function.