route namespace

design notes for #850

I estimate completing the implementation will take half a day.


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UPDATE (from Adam): This took over a day to change it from design to implementation, including hours of pairing, but the estimate was for Jean-Paul alone and I'm slower. There are also a few TODOs with questions about how to continue.

Fixes #850 

@dwgebler and I have been looking at this, and I'm a bit stuck thinking about what is the best option (sorry if I'm being very slow).

`HostNetwork` methods `create_proxy_to` and `enumerate_proxies` require a namespace, so `INetwork` and its implementers should have `namespace` parameters to those methods, and `HostNetwork` should not have a `namespace` attribute. Does that sound right?