Responders: Fix client destructor
To close a socket associated to an fd event we must set the close
function of the event and not associate a destructor to a parent context.
Otherwise the destructor will close() the socket before the fd event is
freed, and this may cause invalid calls on a closed file descriptor to
poll/epoll/etc.
Discovered by looking at strace output.
Resolves:
https://fedorahosted.org/sssd/ticket/2973
Reviewed-by: Pavel Březina <pbrezina@redhat.com>