Asks Mixpanel to send what it has. It doesn't say when it's done.
Forgets the current user. Call it on sign-out.
Flushes before opting out, because opting out deletes what Mixpanel hasn't sent.
Mixpanel is ready once init has finished, so there is nothing to do.
Mixpanel's start, sign-in and consent.
Create Mixpanel opted out, so it sends nothing before consent.
startcan't opt out for you: that deletes unsent events and the stored user every launch.