Improving offline / intermittent connectivity behaviour

This release address a particularly nasty issue with how MixpanelClient handled offline / poor connectivity.

Specifically, if you were offline or unable to actually reach Mixpanel, this background thread sending to the service would spin forever, consuming a whole CPU core.

This address that by adding a back off on the retry, which will ultimately stop processing items if the network is unavailable. If the network is actually offline, it will resume processing on the next network state change. Otherwise it will wait for the queue to be restarted for other reasons -- resume after suspend, app restart etc.