[ecash-herald] Standardize headers for msg sections

Summary:
T2972

Standardize headers for msg sections. All headers should have bold titles, include a count, be bolded, and not end with ':'

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14177