When an error gets reported to HistoryBackend::DatabaseErrorCallback(), we post a task to HistoryBackend::KillHistoryDatabase that closes the db and sets db_ to nullptr which protects against further DB operations.

This allows other tasks to access the DB in the meantime (before KillHistoryDatabase gets run). This is dangerous.

We should
 - either block further tasks from accessing the DB or
 - kill the database immediately.