PG: clarify same_primary_since updates regarding primary rank

pg_shard_t includes the position, so these checks handle
same osd/different rank properly.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Objecter/OSDMap: factor out primary_changed() into static OSDMap method

We need to reuse this logic in OSD::project_pg_history.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD::project_pg_history needs to account for acting_primary/up_primary

Fixes: #7512
Signed-off-by: Samuel Just <sam.just@inktank.com>