[Chronik] Add ParseChronikParams and use it in chronik::Start

Summary: This separates this code into its own function, something we eventually could test also.

Test Plan: ninja check-functional

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16388