osd/ReplicatedPG: skip missing hit_sets when loading into memory

We weren't handling hit_sets that were missing.

Two changes here:

1- Load the hit_sets oldest to newest.  That means that if we stop partway
   through loading, and then add another to the end of the list, and then
   try again to load some more, we will still catch them all.
2- If the object is missing, stop.  We'll try again the next time
   agent_work() is called.

Fixes: #8077
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1655 from ceph/wip-8077

osd: handle missing hti_set objects in agent_load_hit_sets

Reviewed-by: Samuel Just <sam.just@inktank.com>