[v3] Add option 'persistence' and make setPersistence internal

[moooji]

setPersistence has to be called before any database references have been created. Since the database constructor immediately creates refs to .info/serverTimeOffset, calling setPersistence will always throw and it is not possible to enable persistence currently. Furthermore, setPersistence is not part of the Web SDK API anyway.

To fix this issue #216, this PR makes setPersistence internal and adds instead an option persistence that can be supplied to the Firestack constructor. This guarantees that persistence will be enabled before any refs are created. The default value is false.

const firestack = new Firestack({ persistence: true }); 

[Salakar]

We use v3 in our prod app as it currently is, with persistence enabled and we don't face this issue, don't think its this causing the issue?

[moooji]

@Salakar I continue the discussion here so that we do not have 2 separate threads going on #216

[Salakar]

@Ehesp , can you confirm, we've notnhad this issue right?

[Ehesp]

Commented on other issue. Might as well close this.

[auser]

I really like this update. Will continue with the other thread.

[benadamstyles]

@auser Do you think this PR could be merged, or did you close it because you don't think it should be? There's a lot of love for it over in #216 !