osdc: implement Objecter::set_epoch_barrier

For use in MDS and Client when they want to wait
for a particular OSDMap epoch before using caps
that depend on a prior blacklist or op cancellation.

Signed-off-by: John Spray <john.spray@redhat.com>
fixup: mds: use objecter epoch barrier instead of holding up cap messages

... so that we don't break the ordering on mds session messages.

Squash into: "mds: implement osd epoch barrier on caps"
fixup: client: use objecter epoch barrier

squash into: "client: track osd epoch barrier"

Signed-off-by: John Spray <john.spray@redhat.com>