[chronik] cleanup of json_to_u31 helper function

Summary:
This makes the code more idiomatic, saves a string conversion (in the success case) and two clones in all cases.

Depends on D17501

Test Plan: nija check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17507