[Chronik] Add ChronikBridge::find_fork, for CChain::FindFork

Summary:
Walks back the chain until it finds a common ancestor.

Depends on D13289.

Test Plan: ninja test_bitcoin && ./src/test/test_bitcoin -t chronikbridge_tests

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13290