Updated version
Use persist method instead of events

- Strip out the use of EventEmitter and user a persist method which
will be overwritten. This is an experiment to try and fix the issue
with some username not being saved to the db despite being logged.

- Use eachSeries instead of each. This was done to try and make the
code more robust by saving a single user at time. It’s likely a bit
slower than each but it’s not critical in this case.