Rename `NodeId` and `NodeAddr` to `EndpointId`, `EndpointAddr`

[flub]

Because of historical reasons we have an Endpoint identified by a NodeId. Because we struggled with this a lot in the past, the usage of these terms is very consistent, also internally in the code. Though it might not be the most obvious for new users and could evoke the wrong paradigms to think about.

[b5]

After discussion we've settled on iroh::EndpointId and iroh::EndpointAddr as the renames for Node*