This PR does a few things:
1) Add a call GetContactCode to check whether we have a bundle for a
given user.
2) Add a DH flag to the API (non-breaking change), for those messages
that we want to target all devices (contact-requests for example).
3) Fixes a few small issues with installations, namely if for example a
messages is sent without a bundle (currently not done by any client),
we still infer installation info, so that we can communicate securely
and making it truly optional.
Andrea Maria Piana