RemoteAsyncDataProvider should not re-fetch between xDS refreshes

Currently, LDS update causes a fresh fetch of a Wasm binary. This is unnecessary since the resource comes with a content hash for verification. The request is to make it possible that RemoteAsyncDataProvider caches whatever it fetches and hands it off between xDS resources (e.g. a warming listener should pick it up from a draining listener).

/cc @yxue
/cc @jplevyak