Offline support for React Native SDK

[Mercy811]

Summary

This issue is to collect customer requests on supporting offline mode for React Native SDK while Amplitude Browser SDK supports offline mode out of the box by #644.

Leave a comment under this issue with the use causes when and why you need this feature. We will prioritize it and fit into our roadmap.

[cgibson-emesent]

This would be a very useful feature for React Native as it's more likely to go "offline" then other use cases. E.g, a user is using the app on a train and the 4G reception drops out (either bad area or going through a tunnel). So those events will be lost.

For our use case, the app is used in locations where internet connections are almost guaranteed to be non-existent (underground or remote areas). The phone / tablet connects to our devices WiFi to stream data and once they have completed their scan will then connect to a internet based WiFi.

Being able to keep the events in local storage until there is a successful upload of the event would be vital to us. As in, if the sending of the event fails due to a network issue, it should not be removed from the unsent collection and be available for another retry.