librbd : v3 copy-on-read for clones

modified names "COR" , "cor" and "if ("  to keep the same code style
AioRead object(in clone) doesn't exist before copy-on-read, so it can't be destoryed. 
m_entire_object is read from parent, no refercing object in clone, 
only after it writen back to clone, m_entire_object refereces AioRead object