[herald] Abbreviate some replayed memo msgs

Summary:
Most memo txs are replayed on BCH, and some are clearly not XEC related.

Abbreviate these in ecash-herald.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15114