[herald] Create msg lines while iterating over parsed txs

Summary:
T2972

Since app was built up incrementally, this inefficiency was overlooked.

parsedTxs has all the info necessary to create a tg msg line. It doesn't make sense to iterate over parsedTxs to create more parsed info arrays, then iterate over those to create msg lines.

Create msg lines while iterating over parsedTxs. Then organize them.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13697