[ecash-herald] Send daily summaries covering exact 24-hr windows

Summary:
We get better information getting on chain activity by timestamp instead of block. In this way, we can tell if blocks are coming "too fast" or "too slow."

We also are always covering a fixed timespan, so daily messages are actually daily messages. So, we can learn more by comparing them against each other.

Test Plan: npm test, node scripts/getDailySummary.js

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16914