Ceph

I discussed this on irc and it's not clear if we want to provide any guarantees or not, but it's certainly unpleasant to find that bufferlists we pass in are randomly cleared by the librados internals (in eg, IoCtx::write_full()) in a way that has no relationship to when the underlying memory is released. In my demo I was trying to use a base bufferlist, and do several writes using that bufferlist with a little extra tacked on to the end; perhaps that's enough of a toy to be not worth supporting but I feel like it generalizes a little bit...