OnionMessenger::new will now take a custom onion message handler trait
implementation. This handler will be used in upcoming commit(s) to handle
inbound custom onion messages.
The new trait also specifies what custom messages are supported via its
associated type, CustomMessage. This associated type must implement a new
CustomOnionMessagesContents trait, which requires custom messages to support
being written, being read, and supplying their TLV type.
Adds the boilerplate needed for PeerManager and OnionMessenger to work
together, with some corresponding docs and misc updates mostly due to the
PeerManager public API changing.
Also update the fuzz ChaCha20Poly1305 to not mark as finished after a single
encrypt_in_place. This is because more bytes may still need to be encrypted,
causing us to panic at the assertion that finished == false when we go to
encrypt more.
Also fix unused_mut warning in messenger + add log on OM forward for testing
