messages/MClientCaps: add osd_epoch_barrier

To advise clients to wait for this epoch before
using capabilities.

Signed-off-by: John Spray <john.spray@redhat.com>
mds: maintain osd_epoch_barrier state

So that when a client has sent us a barrier epoch,
we remember it and pass it on to other clients when
handing out capabilities.

Signed-off-by: John Spray <john.spray@redhat.com>
osdc/Objecter: always sub to osd map when full

Even though objecter only pauses IOs if honor_osdmap_full,
we are still interested in subsequent maps when we see
the full flag, for e.g. the MDS applying its own policy
based on the full flag.

Signed-off-by: John Spray <john.spray@redhat.com>
carry around osd barrier epoch

FIXME split up commit