Client::whenResolved() should automatically attach a reference.

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.

c++/src/capnp/capability.h

@@ -882,7 +882,7 @@ inline typename T::Client Capability::Client::castAs() {
   return typename T::Client(hook->addRef());
 }
 inline kj::Promise<void> Capability::Client::whenResolved() {
-  return hook->whenResolved();
+  return hook->whenResolved().attach(hook->addRef());
 }
 inline Request<AnyPointer, AnyPointer> Capability::Client::typelessRequest(
     uint64_t interfaceId, uint16_t methodId,