OSDMap: bump snap_epoch when adding a tier

When we make an existing pool a tier, we start copying the snap metadata
from the base tier.  That includes removed_snaps.  In order for the OSD
to recognize that this value is changing for the first time, we need to
set snap_epoch, or else the OSD doesn't update it's in-memory PGPool
with removed snaps and we eventually hit an assertion failure because
PGPool::cached_remove_snaps is incorrect (e.g., empty).

Fix this by bumping snap_epoch when we add the new tier.

Fixes: #7915
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1591 from ceph/wip-7915

mon: bump snap_epoch when adding a tier (fixes 7915)

Reviewed-by: Samuel Just <sam.just@inktank.com>