mds: set epoch barrier on transition to active

To handle case where MDSs restart after experiencing
a barrier-inducing operation: rather than persisting
OSD barrier somewhere, just have the MDSs always barrier
on latest OSD epoch at startup.

Effect is that after restart, MDS cap issues will
be delayed in (compliant) clients until the client
sees the latest OSD map.

Signed-off-by: John Spray <john.spray@redhat.com>
f
fixup "client: track osd epoch barrier"

(calling got_mds_push twice!)