Use a message buffer abstraction in OnionMessenger

Use a message buffer abstraction in OnionMessenger

Onion messages are buffered for sending to the next node. Since the
network has limited adoption, connecting directly to a peer may be
necessary. Add an OnionMessageBuffer abstraction that can differentiate
between connected peers and those are pending a connection. This allows
for buffering messages before a connection is established and applying
different buffer policies for peers yet to be connected.