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 remote-tracking branches 'origin/wip-9179', 'upstream/wip-hitset-bytes' and 'upstream/wip-9193' into wip-sam-testing