[herald] Patch bug in unidentified miner

Summary:
Depends on D15083

This miner was identified as Hathor-MM following a bug from an external site. While the string 'mm' does regularly appear in these blocks, we don't really know what it means.

We do though know these blocks come from the same miner, since they all go to the same payout address. So, call it something consistent that reflects unknown nature.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15113