#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
Actually, do we need to wait until we have the map to process this? Or can we just increment our osd_epoch_barrier appropriately?
(I guess maybe that'd be an attack that a malicious client could employ to prevent other people getting caps, but....)