Now explicitly closed connections on the client side are
closed with a clean error code. This is motivated by upcoming
work to detect broken websockets and automatically reconnect
Improve the boundary between RpcImpl and LaminarInterface such
that the Laminar class doesn't require any types from kj/async.h.
The necessary logic moved from Laminar to RpcImpl and the notification
now happens by abstract virtual callback instead of kj::Promise.
Also remove the fairly useless 'wait' RPC call and drop the wrappers
around kj::PromiseFulfillerPair
