A rehandshake is not a real failure and should not result in the session
being removed from the session cache. This is causing the client to fail
to resume the session, causing the server to request the client cert
again for rehandshakes, which is undesirable.

When refactoring this code, I wasn't sure exactly where failed() should
be called. I noticed it was not being called in many places where it
really should have been, and overcorrected.

Note that this also moves failed() to be a vfunc of GTlsConnectionGnutls
instead of GTlsConnectionBase, since it's no longer called from Base and
OpenSSL doesn't implement it anywhere.

Fixes #84