Break out URL handlers into objects keyed by msg.type

[michaelfig]

I have a plan to break out the handlers into objects with properties identifying what msg.type field they apply to. At that point, we can keep one handler per URL, which will be an object keyed by the msg.type pointing to its latest installed implementation.

Originally posted by @michaelfig in https://github.com/_render_node/MDIzOlB1bGxSZXF1ZXN0UmV2aWV3VGhyZWFkMjQzNzIxODk1OnYy/pull_request_review_threads/discussion