librbd : v3 copy-on-read for clones

modified names "COR" , "cor" and "if ("  to keep the same code style
Why AioRead object cannot be destroyed? After copying the read content to the buffer, I can see it will be destroyed. "m_entire_object" does belong to the parent object (AioRequest) of AioRead, but  AioRequest will also be destroyed. If asynchronous writing back is still pending, m_entire_object points to a invalid memory.