mds: remove inc array from mdsmap

mds: remove inc array from mdsmap

Instead, use the MDSMap epochs to provide the `inc` that
we populate into mds_info_t for use by the daemons.

It is not necessary to respect the current value of inc
in old maps, because the per-rank counters were necessarily
lower numbers than the overall map epoch (incrementing
the counters always corresponded to a map change).

Fixes: http://tracker.ceph.com/issues/15399
Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 30a375ebf814245fbbb5155fd39d8ff673f0a32a)