identify should not allow already identified users

[marandaneto]

Description

Relates to PostHog/posthog-android#108
Relates to PostHog/posthog-js#524

const isKnownAnonymous = this.persistence.get_user_state() === 'anonymous' is the check where we know if calling identify again is allowed or not.
If the user is already identified, calling identify again isn't allowed, if that's intentional, you should call alias instead.

If you want to identify another user again, you have to call reset first.
https://posthog.com/docs/product-analytics/identify#3-reset-after-logout

There are more restrictions described here.

By not doing this, we might be merging users wrongly and we'll likely get a support request to split persons.

This has to be done on the other SDKs as well such as:

• iOS
• RN (js-lite)
• Android

[marandaneto]

Ideally done with #172