Enqueue onion messages in handlers

Enqueue onion messages in handlers

When constructing onion messages to send initially (opposed to replying
to one from a handler), the user must construct an OnionMessagePath first
before calling OnionMessener::send_onion_message. Additionally, having a
reference to OnionMessener isn't always desirable. For instance, in an
upcoming commit, ChannelManager will implement OffersMessageHandler,
which OnionMessenger needs a reference to. If ChannelManager had a
reference to OnionMessenger, too, there would be a dependency cycle.

Instead, modify OffersMessageHandler and CustomOnionMessageHandler's
interfaces to include a method for releasing pending onion messages.
That way, ChannelManager may, for instance, construct and enqueue an
InvoiceRequest for sending without needing a reference to
OnionMessenger.

Additionally, OnionMessenger has responsibility for path finding just as
it does when replying to messages from a handler. It performs this when
extracting messages from the handlers before returning the next message
to send to a peer.