osd/ReplicatedPG: flush snaps immediately

Snaps are immutable, so there is no point in delaying flush and hoping
to capture additional updates first.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2885 from ceph/wip-flush-snaps

osd/ReplicatedPG: flush snaps immediately

Reviewed-by: Samuel Just <sjust@redhat.com>