SERVER-10670 On a failure resulting from programming error, throw out the RoleGraph.
SERVER-10670 Make isAuthEnabled a member rather than static function of AuthorizationManager.

It should have been from the beginning, and fixing it makes it easier to
write tests.
SERVER-10670 Reenable auth unittests.

This fixes the authorization_session_test and all but two cases in the
authorization_manager_test.  The two failing cases are labeled with a TODO and
disabled individually.