ReplicatedPG: make_snap_collection when moving snap link in snap_trimmer

Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
ReplicatedPG: correctly handle new snap collections on replica

Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>