bindings/s2n-tls: wakers always required, even when async not used

[jmayclin]

Problem:

When using certain callbacks, we force wakers to be set on the connection even if no async polling is happening. This is a confusing customer experience, as customers reasonably expect that they shouldn't need to set wakers if they aren't using any async functionality.

See further discussion in #4596 (comment)

Solution:

At a high level, we should check if the future is None before attempting to retrieve the waker. That way the waker is only required when it is actually used.

Requirements / Acceptance Criteria:

Customer's that aren't using async functionality should not need to set a waker on the connection.