[cpp] Provide a way to query proton::connection for the url it was created with

This is actually somewhat more serious than it appears as it makes trivial C++ examples unintuitive.

That is: You want to just print connection.url() to identify a connection in a trivial connection handler, but you have to print either connection.virtual_host() or connection.container_id(). And both of those need more explanation then is useful in the very earliest code samples.

Another solution to this would be to have operator<<(ostream&, proton::connection&) produce something useful - this would actually be better in the context of a trivail example, but has less real use.