Downloading an action should be part of container initialization

Today, the invoker downloads the entire action (including its code) when a message arrives from Kafka. To the user, that appears as `waitTime`. Note that this time could in theory even occur on a warm invocation if the cache is flushed in between invocations (unlikely, but possible).

With the work @dubeejw was doing in switching everything into attachements, we should follow up once that is merged to download the code only as part of the initialization process (thus it will appear as `initTime`) and is guaranteed not to happen as part of a warm invocation.

With that change in, we could also think about not downloading anything at all if no initialization is necessary. All data we need for a warm invocation should already be part of the `ActivationMessage` (I'd need to doublecheck if that is true).