[Chronik] Add minimal chronik::Start and chronik::Stop to init.cpp

Summary: Current implementation only logs. It gives us a scaffold to add the actual indexer.

Test Plan:

1. ninja
2. ./src/bitcoind -regtest prints "Starting Chronik..." in the logs
3. Ctrl+C to terminate bitcoind prints "Stopping Chronik..." in the logs

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11807