journal: rework terminology

journal: rework terminology

Let's clean up our terminology a bit. New terminology:

FSS = Forward Secure Sealing
FSPRG = Forward Secure Pseudo-Random Generator

FSS is the combination of FSPRG and a HMAC.

Sealing = process of adding authentication tags to the journal.
Verification = process of checking authentication tags to the journal.

Sealing Key = The key used for adding authentication tags to the journal.
Verification Key = The key used for checking authentication tags of the journal.
Key pair = The pair of Sealing Key and Verification Key

Internally, the Sealing Key is the combination of the FSPRG State plus
change interval/start time.

Internally, the Verification Key is the combination of the FSPRG Seed
plus change interval/start time.