* Only fetch block headers from the database to answer headers requests
* Move writing to the database to a blocking thread
* Add blocking threads to tests that need them
* Remove mempool downloader requirement for a populated state
And stop populating states that don't need it,
so we can use tokio::time::pause() in those tests.
* Improve debugging for an intermittent test failure
* Try to avoid a race condition populating the mempool in tests
394d16a5a5