reading into a static buffer is a race condition that is only
manifested under load. There's no guarantee the clause in then()
will run before another task overwrites the buffer. Allocating
a local string is the only correct solution
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
Oliver Giles