unify ContactStore save and update

[tomholub]

This PR makes no difference between save and update operations. I left them untouched so we won't need to update a lot of client code in this massive PR.

Originally posted by @rrrooommmaaa in #3445 (comment)

[rrrooommmaaa]

The only useful feature of save is that we can save multiple contacts in one call,
whereas update allows to apply the same update request to a bunch of contacts.

[tomholub]

I see - that means there is some difference. We should maybe just update the comments of both methods, to say that the purpose of save is meant to add new contacts, however it will also update existing contacts.

And the purpose of update is to update one or more contacts at once, but it would also create a new contact if it was missing 🤷

At least if the oddity is documented, it becomes less odd.