containers: Cancel async destruction from ContainerClient if a new ContainerClient takes over, and be able to recover container sidecar proxies by @gabivlj in https://github.com/cloudflare/workerd/pull/6201
container: Make the canceller identity depend on an autoincreasing integer instead of a pointer, and move ContainerCleanupState as a private class by @gabivlj in https://github.com/cloudflare/workerd/pull/6221