rpc will often need to transfer data received on push-bashed interfaces
to consumers that expect input stream sources, which are pull-based. we
want to avoid copies for performance reasons (since kj overhead as kind
of on the high side for us already), so we'll use a shared-buffer class
that behaves much like a mutex. we *don't* use mutexes because ours are
cross-thread-capable and thus require syscalls for all wakeups they do.
Change-Id: I3b14925f5d9f5e07ea2cafdf00a88a64f79e4742
eldritch horrors
9bd9947f41