fix(ext/cache): acquire reader lock before async op

[marcosc90]

The following snippet should throw instead of consuming the body

  const cacheName = "cache-v1";
  const cache = await caches.open(cacheName);

  const response = new Response('consumed');

  const promise = cache.put(
    new Request("https://example.com/"),
    response,
  );
  console.log(await response.text()); // consumed

[lucacasonato]

LGTM!