Capability::Clients do not follow the usual KJ style with regards to lifetimes of returned promises. RPC methods in particular automatically take a reference on the capability until the method completes. This makes some intuitive sense as Capability::Client itself is a pointer-like type implementing reference counting on some inner object.

whenResolved() did not follow the pattern, and instead required that the caller explicitly take a reference. I screwed this up when using it, suggesting that it's pretty unintuitive. It's cheap and safe to automatically take a reference, so let's do that.