[chronik] make the maximum history length for electrum RPCs an init param

Summary: This will allow server operators to decide what the reasonable limit should be, and potentially increase it if they are not worried about DDos attacks.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17681