[Chronik] Relax plugin height checks at startup

Summary:
Currently, we require all plugins to be synced for every block since genesis. However, this is overkill for most plugins, as most have the first usage of their LOKAD ID much later.

In this diff, allow plugins to load if there's currently no txs that would apply for it indexed in the DB already.

Depends on D16568.

Test Plan: cargo test -p chronik-indexer && ./test/functional/test_runner.py chronik_plugins_setup

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16560