Bug description

When someone logs out / switches to a new browser and then tries logging in, they get a new distinct ID, which can mean that sometimes the feature flags can flip, because feature flags are based on distinctID, not personID.

Then once they're logged in and move to their existing distinctID, until the /decide call returns, we're using the 'wrong' feature flag value. Further, if they keep using the $anon_distinct_id, our feature flag is permanently flipped, which is not great for experiments.

Additional context

1. While there's not much we can do when a new unidentified user pops up, is there something we can do to minimise lag between logging in & new feature flags being loaded?
2. What are problems / advantages with moving to personID as feature flag identifier, for cases when you'd want multiple distinctIDs belonging to the same person to go to the same bucket.

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