OSD: snap collections can be ignored on split

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
PG: clear snap_collections on upgrade

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
OSD: handle stray snap collections from upgrade bug

Previously, we failed to clear snap_collections, which causes split to
spawn a bunch of snap collections.  In load_pgs, we now clear any such
snap collections and then snap_collections field on the PG itself.

Related: #4927
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Merge branch 'wip_split_upgrade' into next

Fixes: #4927