CLEANUP: backend: clarify the cases where we want to use early data

CLEANUP: backend: clarify the cases where we want to use early data

The conditions to use early data on output are super tricky and
detected later, so that it's difficult to figure how this works. This
patch splits the condition in two parts, the one that can be performed
early that is based on config/client/etc. It is used to clear a variable
that allows early data to be used in case any condition is not satisfied.
It was purposely split into multiple independent and reviewable tests.

The second part remains where it was at the end, and is used to temporarily
clear the handshake flags to let the data layer use early data. This one
being tricky, a large comment explaining the principle was added.

The logic was not changed at all, only the code was made more readable.