SSL.File: Improved handling of user callbacks in set_backend().

Any pending callbacks now migrate to the new backend.

Also clears pending callbacks on shutdown().
SSL.File: update_internal_state() is no more.

As update_internal_state() has become a noop, there's no need
to keep it around anymore.
SSL.File: The got_extra_read_call_out state is no more.

The extra read call_out has been fully replaced by the
internal_poll() call_out, so remove the last remnants of it.
On systems with RDRND random will not be deterministic or depend on random_seed().