Get the messaging system up-to-speed with framework model change

[rostgaard]

The message model have changed the MessageRecipient from a "link" to a contact to a dereferenced endpoint. A MessageRecipient may be created from a MessageEndpoint and DistributionList entry.

An example of the usage can be found in the integration tests[1].

[1] https://github.com/Bitstackers/OpenReception-Integration-Tests/blob/a1b1f7a0a36277e2ca1ea9e5d351e2a6be125082/lib/messagestore/src/message_store.dart#L28