[Chronik] Wipe Chronik db when -reindex is set

Summary: Currently, the only way of forcing Chronik to reindex is to delete the indexes/chronik folder. With this change, Chronik now respects -reindex, and uses RocksDB's DestroyDB function to wipe the DB cleanly.

Test Plan: ninja && ninja check-crates && ./test/functional/test_runner.py chronik_reindex

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13315