Implement Method for Changing the member key

This is for 2 reasons:

1. Users can chose their own key which is a personalization feature
2. A given Member key that would be bound to a membership can be anonymized

created merge request !1856 (merged) to address this issue

mentioned in merge request !1856 (merged)

closed with merge request !1856 (merged)

mentioned in commit 4a2e4617