Currently, if a plugin desyncs with the DB, nothing happens, but this could lead to an inconsistent state of Chronik, e.g. where a plugin seems to be up-to-date but actually skipped a few blocks resulting in wrong data being reported to the user.
To avoid this, we previously already added a field `sync_height` to store the plugin's synced height, but it was unused. In this diff, we enforce that all plugins must be synced at startup, or instruct the user to either reindex Chronik or disable the plugin.
In the future, we can automatically reindex only the transactions that are relevant for the plugin using the LOKAD ID index, but since this is non-trivial this will be implemented later. Prioritizing getting a plugin system with mediocre UX out sooner.
Depends on D16532.