Be consistent in SCT/STH acronym use

[richsalz]

Make sure it's spelled out the first time it's used.
Use acronym in section title except for the definition and include
the acronym then.

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[paulwouters]

looks good to me, and I think this can be pulled in during RFC-Editor work or it can be merged in a new draft after our IETF 111 meeting if anything else comes up. Thanks Rich !