Remove adapter-specific message types from automerge-repo

[HerbCaudill]

automerge-repo currently exposes two union types for messages:

• RepoMessage includes all message types that a network adapter emits to the Repo.
• Message includes all of these plus ArriveMessage and WelcomeMessage, which are message types used by some network adapters but not others.

In anticipation of the AuthProvider work, I'm introducing an AuthMessage type, which doesn't fit into either of these categories: It's not Message, because all network adapters need to be able to handle it, not just the ones that have arrive and welcome; and it's not a RepoMessage because it's not surfaced to the Repo.

The abstract NetworkAdapter needs a message type that automerge-repo expects every adapter implementation to accept, and that should be Message.

This PR removes arrive and welcome messages from automerge-repo, leaving it entirely up to each network adapter what kind of internal messages to use. That means that we have some duplicate type definitions between MessageChannelNetworkAdapter and BroadcastChannelNetworkAdapter but that's OK.

Message is redefined as RepoMessage | AuthMessage, and each adapter extends that type with its own internal message types.