This is the TLS1.3 key schedule. It stores the current secret and
the type of hash. This isn’t used directly; but only through the
typestates.
KeySchedule for early data stage.
KeySchedule during handshake.
Pre-handshake key schedule
KeySchedule during traffic stage. All traffic & exporter keys are guaranteed
to be available.
KeySchedule during traffic stage, retaining the ability to calculate the client’s
finished verify_data. The traffic stage key schedule can be extracted from it
through signing the client finished hash.