Find DM group by peer address or inbox id

[nplasterer]

Add the ability to find a dm group by inbox id so that we can dual send messages to V2 conversations and V3.

In V2 world conversations are found exclusively by the peer address since there can only be 1 one to one conversation. It would be nice to be able to closely emulate that until we get conversation threading fully implemented maybe just return the first group dm for the 2 people.