[ecash-herald] Patch bug in tracking per-token agora volume

Summary: We were over-writing the 'volume' key each time this function was called, leading to under-reporting of per-token volume counts.

Test Plan:
npm test, check screenshots.

before:

after:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17164