Cache events

[adammeaney]

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Makes notification action consistent between platforms.

Changes Proposed in this pull request:

• Allow delayed subscription for the opened event for iOS to follow how Android does it.
• Follows a similar pattern for the OnTokenRefresh. This can be removed if you see a problem with it that I do not, but it allows for auto registering at the beginning and only using it after a sign in or similar approaches.

[thomasgalliker]

Is anyone going to merge and release this feature soon? I think the current implementation leads to lot of confusion (Android handles caching while iOS does not).

[thomasgalliker]

You can find an all new implementation with caching of push notifications here.