RBD readahead

I generally like the idea of putting this in ObjectCacher, but it's not 
quite the right fit because ObjectCacher deals with objects *after* the 
striping is applied.  we'd need to make sure that RBD and the cephfs 
client are using the same set of read methods (there might be wrappers 
exposed by ObjectCacher that do the Striper stuff? I forget)...