PG: wait until we've build the missing set to discover_all_missing

Fixes: #9179
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2435 from athanatos/wip-9179

PG: wait until we've build the missing set to discover_all_missing

Reviewed-by: Sage Weil <sage@redhat.com>