[ecash-herald] Organize token send txs by token

Summary:
We do not get a lot of useful information by seeing every token send tx on its own line. It's more interesting (and easier to parse) if we see a summary of txs for each token.

Implement.

Test Plan:
npm test

aesthetic judgment on before:

vs after:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16752