r2648: - use a destructor on struct server_connection to simplify the

r2648: - use a destructor on struct server_connection to simplify the
  connection termination cleanup, and to ensure that the event
  contexts are properly removed for every process model

- gave auth_context the new talloc treatment, which removes another
  source of memory leaks.
(This used to be commit 230e1cd777b0fba82dffcbd656cfa23c155d0560)