[ecash-herald] Group Cashtab rewards separately

Summary:
We may typically have dozens of these in a block. No value is added by showing them all individually, as they are identical.

Group them for best use of msg space.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16018