auth: make connection_secret a std::string

auth: make connection_secret a std::string

Move connection mode decision to initial auth_request point so that it
can inform auth implementation how big the connection secret should be.
Pass that value through where appropriate.

The connection_secret is now a std::string filled with random bytes.

For now the v2 protocol just uses the session_key CryptoKey to encrypt,
but this is about to change.

Signed-off-by: Sage Weil <sage@redhat.com>