[Chronik] Rename setup_bridge to setup_chronik

Summary: This method actually sets up a Chronik instance, not a bridge, so this name is more appropriate.

Test Plan:

1. ninja
2. ./src/bitcoind -regtest -> prints "Starting Chronik..."

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12046