#7317 ENOSPC and OSD epoch barrier handling


Related:
https://github.com/ceph/ceph-qa-suite/pull/225
https://github.com/ceph/ceph-client/pull/7
I'm a little concerned about this function in that it (of course) doesn't cancel writes which have already gone across the wire, so we could actually commit some of them to disk despite returning ENOSPC to the writers. Do the specs allow this, or are we just doing it anyway because we have to, or...?