Tree listener for WebSockets (NNG_OPT_WS_TREE)

[EvanBalster]

This came out of a discussion with @gdamore on Discord. I probably don't need this feature for my own application; specifically, I am writing a request router for persistent connections with its own trie structure.

Currently there's a feature in NNG where HTTP microservices can respond to a "tree" of URIs. I believe this could also be useful for some use-cases with WebSockets.

Garrett suggested this option would take the form of an option "NNG_OPT_WS_TREE", indicating that dialers to a sub-path should connect to the (longest) tree which is a prefix of that sub-path.

For example, a server might host the following WebSocket paths, all as trees:

/orchard
/orchard/fruit
/orchard/fruit/apple
/orchard/fruit/orange
/orchard/nut
/orchard/nut/almond

If I were to request /orchard/fruit/banana, I would connect to /orchard/fruit which is the longest existing parent path. This would allow for a sort of fallback from specific services to service classes.