* EventIndex: add some documentation
... because comments are not subject to rationing.
* EventIndex: rationalise addInitialCheckpoints logic
* EventIndex: improve logging
* use a single `logger` instance to do the prefixing
* use `JSON.stringify` on the checkpoints so that the rageshakes are useful
* distinguish between logger.warn and logger.debug
* emit some logs if the crawler loop fails
* Move check for empty database to `init`
The problem with checking if the database is empty in `onSync` is that, by the
time we get there, it won't be.
Instead let's remember if the db was empty in `init`, and then once a sync
completes, we can add the checkpoints.
* Some tests for EventIndexer
2fb0bf6152