[ecash-herald] Restrict length of unknown op-return msgs

Summary:
Intended impact of printing hex for non-ascii OP_RETURN msgs without a known parsing schema was to improve aesthetics of msgs. This isn't done when the unknown msgs are also very large.

In such cases, abbreviate the msgs.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14479