saml: Split statsData array out from statistics logging.

This change makes it easier to add elements to the statistics data
logged when the IdP sends an authentication response.
saml: Add total time of login operation to stats data.

Add statistics data to saml:idp:Response and saml:idp:Response:error
with the total time since we began processing the authentication
request.
Merge branch 'feature/saml-idp-total-login-time'