Proposal: change Router and Worker to specify the "self address"

[hairyhum]

Context:

Currently Ockam.Worker.handle_message/2 receives a message which was routed via Ockam.Router.route/1 and expects a first address in the onward_route to be a "self address", while it also has this address in the worker state.

This is an implicit assumption, which might not hold true. That makes message forwarding harder as the worker has to check the address and remove it from the onward_route on forwarding.

Some internal mechanisms like TCP transport do not use the Ockam.Router.route/1, but send the message directly to the worker, which means there is no "self address" in this message. This is inconsistent with the other workers.

Proposed change:

Change the Ockam.Worker.handle_message/2 callback to accept 3 arguments: (routed_address, onward_message, state) where routed_address is the first addres in the onward_route, which was used in Ockam.Router.route/1
In most cases when the message was routed to the local worker, this routed_address will be the same as address in the worker state.

The onward_message in this case is a message used in Ockam.Router.route/1, but with the routed_address removed from the onward_route.

This way the worker doesn't have to deconstruct the onward_route, doesn't have to assume that the first element there is the same as its address in the state and may have the knowledge about the address it was accessed by even if it's different from the state address.

When forwarding a message, the worker would use the onward_message without modification to the onward_route and only needs to update the return_route (and maybe the payload)

The API for Ockam.Router.route/1 would stay the same, while Ockam.Node.send/2 would have to accept the onward_message as the second argument and pass the first argument as the routed_address to the worker.

The downside here is that the message in handle_message/3 and route/1 would be different. Also the implementation should choose the format for the erlang message to contain the ockam onward_message and the routed_address, which might be Ockam.Message, but then Ockam.Worker would do the job of deconstructing it.