rgw: revisit atomic GET/PUT
Discuss the following different options to simplify the process:
1. Instead of writing tmp object and then clone it to the head object, we'll write the shadow object and just clone the first chunk to the head object. Every read will first go to the head object and continue with the shadow.
2. Similar to 1, just that we don't write the first chunk to the shadow object, instead we write it only to the head object at the end of the write.
Both options will allow us to remove the relatively complicated race protection code that we have now, no need for retries, etc.