hsclient: Rename `get_or_launch_connection` to `get_or_launch_circuit`?

In many cases we avoid using the term "connection" because it tends to be vague about whether we mean a channel, a stream, or a circuit. I just hit this confusion with get_or_launch_connection in tor-hsclient: I was uncertain by looking at the name whether it would give a circuit, a stream, or some weird third thing.

We may want to keep the old name but deprecate it. If not, we should mark this as a breaking change.